kernel functions for sphere and polyprism

Both the doctests and the html in fatiando\doc\_build\html are ok

**Update by @leouieda**:

Checklist:

- [ ] Cookbook recipes for new functions
- [ ] Docstrings
- [ ] Doctests and unitests
- [ ] All tests pass
- [ ] Can be merged
- [ ] Changelog entry
- [ ] Benchmark against old implementation