Add reference to Nagy et al. (2002)

It introduces some fixes to the equations that define the horizontal
components of the gravity acceleration.

doc/references.rst

@@ -9,6 +9,7 @@ References
 .. [Hofmann-WellenhofMoritz2006] Hofmann-Wellenhof, B., & Moritz, H. (2006). Physical Geodesy (2nd, corr. ed. 2006 edition ed.). Wien ; New York: Springer.
 .. [LiGotze2001] Li, X. and H. J. Gotze, 2001, Tutorial: Ellipsoid, geoid, gravity, geodesy, and geophysics, Geophysics, 66(6), p. 1660-1668, doi:`10.1190/1.1487109 <https://doi.org/10.1190/1.1487109>`__
 .. [Nagy2000] Nagy, D., Papp, G. & Benedek, J.(2000). The gravitational potential and its derivatives for the prism. Journal of Geodesy 74: 552. doi:`10.1007/s001900000116 <https://doi.org/10.1007/s001900000116>`__
+.. [Nagy2002] Nagy, D., Papp, G. & Benedek, J.(2000). Corrections to "The gravitational potential and its derivatives for the prism". Journal of Geodesy (2002) 76: 475. doi:`10.1007/s00190-002-0264-7 <https://doi.org/10.1007/s00190-002-0264-7>`__
 .. [TurcotteSchubert2014] Turcotte, D. L., & Schubert, G. (2014). Geodynamics (3 edition). Cambridge, United Kingdom: Cambridge University Press.
 .. [Vermeille2002] Vermeille, H., 2002. Direct transformation from geocentric coordinates to geodetic coordinates. Journal of Geodesy. 76. 451-454. doi:`10.1007/s00190-002-0273-6 <https://doi.org/10.1007/s00190-002-0273-6>`__
 .. [Moritz2000] Moritz, H. (2000). Geodetic Reference System 1980. Journal of Geodesy, 74(1), 128–133. doi:`10.1007/s001900050278 <https://doi.org/10.1007/s001900050278>`__

harmonica/forward/prism.py

@@ -11,7 +11,7 @@ def prism_gravity(coordinates, prisms, density, field, dtype="float64"):
  """
  Compute gravitational field of prisms on computation points.
 
- Forward modelling based on [Nagy2000]_.
+ Forward modelling based on [Nagy2000]_ and [Nagy2002]_.
 
  .. warning::
  The **z direction points upwards**, i.e. positive and negative values of