Fix missing PyGMT images in user guide (#474)

Add hidden cell in user guide examples configuring PyGMT to use
`"notebook"` as display method. This way `jupyter-sphinx` is able to
render the images in the generated HTML files. This issue should be
fixed upstream in PyGMT.

doc/user_guide/equivalent_sources/block-averaged-eqs.rst

@@ -62,6 +62,19 @@ And project the geographic coordinates to plain Cartesian ones:
  coordinates = (easting, northing, data.height_m)
  xy_region=vd.get_region(coordinates)
 
+
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/equivalent_sources/eq-sources-spherical.rst

@@ -146,6 +146,18 @@ grid points that are too far away from any data point:
 
 Lets plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/equivalent_sources/eqs-parameters-estimation.rst

@@ -188,6 +188,18 @@ And grid the data using the two equivalent sources:
 
 Lets plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/equivalent_sources/gradient-boosted-eqs.rst

@@ -145,6 +145,18 @@ point. We can do so through the :func:`verde.distance_mask` function.
 
 And plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/equivalent_sources/index.rst

@@ -112,6 +112,18 @@ sources are able to reproduce the observed field.
 
 And plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/forward_modelling/point.rst

@@ -91,6 +91,18 @@ generated by the whole set of point sources on every computation point:
 
 Lets plot this gravitational field:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/forward_modelling/prism.rst

@@ -185,6 +185,18 @@ generated by the whole set of prisms on every computation point:
 
 Lets plot this gravitational field:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/forward_modelling/tesseroid.rst

@@ -96,6 +96,18 @@ generates on the computation point:
 
 And finally plot the computed gravitational field
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/gravity_disturbance.rst

@@ -63,6 +63,18 @@ These observations are located on a regular grid on geodetic coordinates at the
 same height of 10 km above the reference ellipsoid.
 Lets plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt

doc/user_guide/topographic_correction.rst

@@ -52,6 +52,18 @@ in Southern Africa.
 
 And plot it:
 
+.. jupyter-execute::
+ :hide-code:
+
+ import pygmt
+
+ # Needed so that displaying works on jupyter-sphinx and sphinx-gallery at
+ # the same time. Using PYGMT_USE_EXTERNAL_DISPLAY="false" in the Makefile
+ # for sphinx-gallery to work means that fig.show won't display anything here
+ # either.
+ pygmt.set_display(method="notebook")
+
+
 .. jupyter-execute::
 
  import pygmt