Add magnetic fields of prisms (#17)

Add forward modelling functions for the magnetic field of a single rectangular
prism on a single computation point. These are three functions for each
component of the magnetic field vector and one other `magnetic_field` function
that computes all three in a more optimal way. Make these functions to reuse
the kernel functions defined for the gravity forward modelling. Add symmetry
tests for the new functions, test if the magnetic field has zero divergance and
if the `magnetic_field` function generates the same output as the three other
functions. Rename `prism/_forward.py` to `prism/_gravity.py` and
`test_prism.py` to `test_prism_gravity.py`.

choclo/prism/__init__.py

@@ -7,7 +7,7 @@
 """
 Kernels and forward modelling functions for rectangular prisms
 """
-from ._forward import (
+from ._gravity import (
  gravity_e,
  gravity_ee,
  gravity_en,
@@ -31,3 +31,4 @@
  kernel_u,
  kernel_uu,
 )
+from ._magnetic import magnetic_e, magnetic_field, magnetic_n, magnetic_u

choclo/prism/_forward.py → choclo/prism/_gravity.py

@@ -5,7 +5,7 @@
 # This code is part of the Fatiando a Terra project (https://www.fatiando.org)
 #
 """
-Forward modelling function for rectangular prisms
+Gravity forward modelling functions for rectangular prisms
 """
 import numpy as np
 from numba import jit