Sphinx Docs missing references

The current generated docs have quite a few missing autodoc references. This leads to broken links in different parts of the documentation (e.g. strand type `chr` in [`Interval`](https://deepgenomics.github.io/GenomeKit/api.html#genome_kit.Interval.__init__), should be `str`).

Low priority but could be worth going through to find all missing/invalid references. Sphinx details can be seen with `-n` flag when building docs (to be changed in the Makefile).

Build the generated documentation with Sphinx's -n option, using the Makefile entry point, and review the missing or invalid autodoc references reported there. Check the generated API documentation, including the Interval constructor reference, and finish when the reported references are corrected and the documentation builds without those warnings.

Written by the indexing model from the issue text.