write unit tests for geometry package

[bsipocz]

as mentioned in #99, unit tests are missing for the geometry package . Also check whether circular_overlap_grid is faster than elliptical_overlap_grid, if not then get rid of it. Checked, and it's indeed significantly faster.

Make the geometry APIs uniform.

[astrofrog]

The APIs should now be uniform, but there is still a need for unit tests. We only need to test the circular_overlap_grid, elliptical_overlap_grid, and rectangular_overlap_grid functions

[astrofrog]

These tests should include a comparison to a reference result to make sure that rotation is properly dealt with (otherwise we could get the total area right, but the angle wrong).

[bsipocz]

Tests were added in #276, and the geometry module is about to be moved to the regions package. Any further issues or requests should be reported there.