Allow fitting of arbitary formulas

[zsusswein]

We use RtGam() as the class constructor. In the language of Advanced R, it wraps both the helper and the validator. It does rigorous input checking, builds the dataset, and fits the model. This object can then be passed to summary() to extract draws and get various metrics.

We could enable faster prototyping while continuing to provide access to post-processing machinery by exposing fit_model() and allowing RtGam() to return a prepared but not fit object. This change could allow arbitrary model formulas to be supplied.