Missing docs for traits

There are some docs missing for 
- `ModelSearchCost` (say what `freqs`, `params`, `empirical_freqs` is doing to make it easy to understand)
- `QMatrix` (is `n = q.nrows = q.ncols = #alphabet`?)
- `EvoModel`: why do we need this, and where is it needed?
- ... (this list may not be exhaustive)