Write code for extracting process for running vb mode (Python)

[dlemfh]

In the (normal) case of StanModel.sampling(), a StanFit4Model obj is returned, which has a .extract() function that can be used to extract the parameter values. (This is the usual case for running hBayesDM models, with vb=False.)

However, when running with vb=True, the pystan function that is used - StanModel.vb() - returns a Dict obj (not the StanFit4Model obj as in the usual case of using StanModel.sampling()).

Therefore, a different process of proper code needs to be written to handle the case for when the variational Bayesian method is requested by the user, for the Python hbayesdm package.

(I.e. Currently, the variational Bayesian analysis functionality is not supported in the Python package.)