Create skeleton brmsfit and stanmodel objects for each model

[dbarneche]

This needs to be added to data-raw/sysdata.R

So then one single R/sysdata.rda is created.

[dbarneche]

For this issue, it’d be great to have a list of all models allowed, broken down by family distribution @beckyfisher
This way I can systematically implement the code to create the empty lightweight brmsfit objects with accompanying R functions (issue #12). After those two steps I can then integrate the calls with define_model.R (issue #13)

[beckyfisher]

@dbarneche At the moment all models allowed are currently in define_models and all families should be allowed for each, using their appropriate link function.