Function for planar Bouguer correction

[leouieda]

Adds a bouguer_correction function that calculate the planar Bouguer correction for land and oceans. The topography provided can be a numpy array or xarray.DataArray. Includes an example application to the global gravity sample data. In this case, need to convert the geoid-referenced ETOPO1 topography to ellipsoid heights.

Fixes #8

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.