Move functions in `gravmag.half_sph_shell` to `gravmag.spherical_shell` for a more general module

The new module `fatiando.gravmag.spherical_shell` will contains the full shell effects as well as of the half
shell. This way I can use both against the tesseroids and can also test against some published results.

Checklist:
- [ ] Make tests for new code
- [ ] Create/update docstrings
- [ ] Code follows PEP8 style conventions
- [ ] Code and docs have been spellchecked
- [ ] Changelog entry
- [ ] Documentation builds properly
- [ ] All tests pass
- [ ] Can be merged