check_regression_features unified with check_features, classmethods removed from SDM

This builds on #430 for the SDM package:

1. Remove classmethods from `SDTrainer`. If duplicated check, use check in `fitmutlilevel.checks`. If not repeated, leave as standalone function in the `fitmultilevel.sdm.trainer` package.

2. Refine checks for features. If you pass `pyramid_on_features` **features must now be a single callable and not a list of length one**. Unless I'm missing something this just adds something else to worry about - it's clearer if it's just not allowed to be a list if you aren't allowing multiple features.

3. `check_list_callables(...)` is broken out from `check_features(...)`. This is reused by `check_regression_features` to save repetition.