Enable Weighting Generally Across Models

[tercer]

Design an architecture that allows weighting approaches to be used in models, especially when some such option exists in the wrapped package, but also with unit weighting by dataset adjustment for models without weights as arguments.

[tercer]

Weighting now works generally across all models, including bootstrapping approaches to build datasets when wrapped package does not handle weights. Weights can be vector of values, or variable name, or MatchIt object. Checks of the "weights" argument make sure the user specified weights appear valid.