[Refactor] `Params` class, which wraps around a dictionary and stores all `Param` information

[digicosmos86]

This class also handles the following:

1. Determines and keeps track of which Param to use as parent
2. Iterates over all params and produces Bambi definitions of formula and priors
3. Produces printouts for all parameters
4. Produces user specifications in all parameters for model saving/loading