Add ModelCollection class

[jgallowa07]

@jbloom related to our conversation last Monday.

Because fitting a collection of models is important for hyper-parameter tuning (in particular lasso strength coeff $\lambda$), we should offer class that can store multiple fits, and can produce plots (altair for tracing certain muts?) like the shrinkage and sparsity, loss, and potentially correlation.

Note: a bunch of useful code for merging parameters and plotting exist in the spike analysis.

In addition to the dms vep pipeline I'm imagining a more standalone pipeline for multidms, so hopefully we can parallel-ize these fits using those. @sminot might have a great platform for hosting both such pipelines

[jgallowa07]

Closed with #122