Add intersphinx mapping for hyperlinks to external docs (numpy, scipy, etc.)

The "See Also" section of docstrings now support hyperlinks to numpy, scipy, matplotlib, pandas, and python docs.

For example, `bioenv`'s "See Also" section previously referenced `scipy.stats.spearmanr`. The function name is now a clickable hyperlink to scipy's docs for that function.

Also fixed a few errors in docs while browsing through and verified that existing references to numpy, scipy, and matplotlib are working.

Partially addresses #190.