Maintenance of the Basis class

[rileyjmurray]

This PR makes changes to the Basis class to help with my leakage PR (#410).

Specifics, at time of writing:

• I split the mind-bogglingly-complicated Basis.cast function into four separate functions. Basis.cast dispatches the appropriate function just like before.
• I removed the "classical_label" keyword argument from Basis.cast. It only appeared in a specific codepath, and it turns out that none of pyGSTI's existing code set this to anything other than its default value.

About EmbeddedBasis ...

It turns out that the EmbeddedBasis class isn't used anywhere in pyGSTi. (There are a few places that import it, but they still never use it.) I discovered this when I got an error trying to access eb.elements for an EmbeddedBasis eb. That trigged evaluation of EmbeddedBasis._lazy_build_elements, which tried to call an undefined function. See here for a repo-wide search for that function.

I'd like to use this PR as an opportunity to either (a) make EmbeddedBasis work, possibly with new semantics, or (b) remove EmbeddedBasis altogether.