Explore basic methods for gpboost

[m-clark]

gpboost seems like a viable package for fast mixed models conducted on large data, possibly for nonlinear effects. For standard mixed models, it'd be nice to have the usual summaries for fixed and random effects at least. The rest of it's functionality might be too much to sort out, but if one is only doing what amounts to a glmm, hopefully we could extract some presentable results.