Fix require_numba decorator

[santisoler]

Make function wrapper of require_numba decorator run the decorated function passed as
arguument. Refactor accuracy tests for tesseroids to solve some hidden mistakes produced
by previous untested lines.

Fixes #90

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.