Use cls for classmethod argument

[carlthome]

Minor polish but self is reserved for the instance, so classmethods should not take in self but rather cls like in:

msaf/msaf/base.py

Line 453 in 7fcedb4

def select_features(cls, features_id, file_struct, annot_beats, framesync):

Stems out of seeing warnings from the Python extension in VS Code.

[urinieto]

lgtm, thanks!