Move StatsModel methods to StatsModel

[piever]

IterableTables defines a fallback when trying to use StatsModel formulas with something other than a DataFrame here. For the sake of reducing type piracy, this methods could live in StatsModel instead: one would still need to load IterableTables to convert any iterabletable to a DataFrame, but everything that has a method to be converted to DataFrames would work without needing this package.