fix "Instantiating a Model object directly" warning

[jacobfc]

In the VPfit.initialise_model() method, mc.model is called directly.

As of pymc==2.3.7, this results in a warning:
"Instantiating a Model object directly is deprecated. We recommend passing variables directly to the Model subclass."

Currently this warning is being suppressed, but it would be nice to change the method so that suppressing the warning is no longer necessary.