Condensed gravmag.prism tests into a single func

Looping over field names and using getattr.
Bx component for prism fails in point on vertices

There is a singularity in the formula if the computation point is
aligned with one of the vertices of the prism. The test now uses a grid
that has points that coincide with the prism edges so that it catches
this kind of error.
Prism gravity must be consistent around prism

Implemented a test to check if this passes. Fails for now.
Fix prism around tests and most tests pass

Made safe_atan2 and safe_log functions that correct the arctan2 quadrant
and log(0) limit for the formula. Only test failing now if for tensor
cross-components when points align with the prism edges in some faces.
Fix prism tensor cross-components

There was a singularity when the computation point was aligned with one
the corners of the prism on a certain side. Hacky fix by just moving the
point slightly when that happens. Not the best fit but works reasonably.
Had to reduce the accuracy of tests for these cases.
Removed unused assertions from _prism_numpy

This is just a test module and doesn't need all the assertions for
corner cases.
Merge branch 'master' into prism-around

Conflicts:
	fatiando/gravmag/_prism_numpy.py
	test/test_gravmag_prism.py
Warning in prism docstrings ab corner singularity
Changelog entry for prism gravity fix
Test gravmag.prism gets the right physical prop
Testing for shape mismatch in gravmag.prism
Merge pull request #113 from leouieda/prism-around

Prism gravity consistent around the prism