v0.6.4

This is a minor release with several new features and cleanups.

New features and enhancements:

• Added support for keeping track of the source of variables being used to
  evaluate a formula. Refer to the ModelSpec documentation for more details.

Bugfixes and cleanups:

• All functions and methods now have type signatures that are statically checked
  during unit testing.
• Removed OrderedDict usage, since Python guarantees the orderedness of
  dictionaries in Python 3.7+.
• Suppress terms/factors in model matrices for which the factors evaluate to
  None.