Proposal: better model subsetting / metadata tracking

[cfhammill]

Dealing with *Lm and *Lmer objects is a nightmare for doing anything non-standard. The big issue is attribute tracking and the imperfect mapping of stat-type and df attributes to columns. I think I should overhaul this and replace with a data.frame mapping the metadata. Subsetting the columns becomes row filtering, appending new columns is row binding or insertion.