Added CPreprocessor::apply() and CFeatures::copy_dimension_subset() methods

@karlnapf Please have a look! I added
- `CFeatures::copy_dimension_subset()` method which stays unimplemented in base class but works in CDenseFeatures. Its possible to use this with subsets as well.
- `CPreprocessor::apply()` method. Subclasses use this as a wrapper to handle specific cases.
Mmmh. You are right PRETTY_FUNCTION will do things when not implemented. Sorry. Just always want to make sure there are no cryptic error messages for users that try stuff :)

This warning things and returning NULL is not good. An error should be thrown. Returning NULL will likely result in a crash