tidy classifiers

1. Removed needless `classifier` function (it simply applied the function it was given, as classifiers are callables just like features we can just go ahead and invoke them directly)
2. module rename: `clm.classifierfunctions` -> `clm.classifiers`
3. `classifier_type` -> `classifiers` in `CLM` classes (now consistent with `features` from `feature_type` introduced in #426)
4. Improved documentation for what classifiers concretely are (`callable`s returning `callable`) rather than what they may be (simple function returning a closure, class instance with `__call__` method).

Note that this is designed on top of #432, so the diff will look large until that is first brought in.