Refactor ._fit() code

[dlsun]

The internal fit method (._fit()) should only contain linear algebra on Numpy arrays. Move any parts that involve column names, creating DataFrames, etc. outside this function.

[ajboyd2]

Move lines 321 to 339 in model.py outside of the _fit method + (and lines like 309 and 293).

[dlsun]

This is looking good, but we should also move the column names outside of the function and return the covariance matrix (rather than setting it in self.cov).