[maybe] Refactor Jackknife and CV methods (DRY)

[pat-alt]

Currently we have separate constructors for Jackknife and CV methods, e.g. JackknifePlusRegressor and CVPlusRegressor. Since the former is just a special case of the latter (for which nfold=nobs), technically this makes the former constructor redundant. By getting rid of it we could make the code base more DRY ("don't repeat yourself").

The problem with this idea is that at instantiation the models have no access to data, so nobs is unknown. So if we want to keep a separate model type (JackknifePlusRegressor), then making the code more DRY would come with its own complications.

Currently undecided, so won't fix myself, but leaving this here for discussion.