Automatic setup of models with smooth terms

[osorensen]

This relates also to #74. I need to create functionality that allows the user to define models as the following, and automatically translate it to the underlying lme4 representation.

mod <- galamm(
  formula = y ~ s(x) * loading + (0 + loading | item),
  data = dat,
  lambda = , load.var = , factor = )

[osorensen]

Will need to understand exactly how gamm4 does this, including it's calls to mgcv::interpret.gam and gamm4:::gamm4.setup. The latter is a non-exported function, so I might have to copy the code and then cite it properly.