Change None specification for priors to be Stan default flat priors

[alizma]

Prior selection currently defaults to autoscaled normal priors and doesn't allow for flat priors. Change the API so the default Stan flat priors are accessible.

[alizma]

See commit 561af87. As stated in the updated documentation, a user can access the Stan default flat priors by passing empty dictionaries {} for any of the regression coefficients. The current set-up ensures that this can be done for individual regression parameters while priors can still be specified for other regression parameters.