WIP Topographic correction in spherical coordinates

[leouieda]

Add a function topographic_correction that calculates the effect of topography and water in spherical coordinates using tesseroids. The function converts the topography from geodetic (the most common input type) to geocentric spherical. The conversion requires the data to be interpolated back on a regular grid (using nearest neighbours by default).

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.

[leouieda]

Work in progress still. Need to have fatiando/verde#225 first to be able to do some of the conversions. The reference level also needs to be converted to spherical on the same points and the interpolated topography.

[leouieda]

Closing this since we're still deciding what to do about topographic corrections.