Make access to primitives more natural

[epeters3]

Currently, all primitives in the package must be accessed by key through primitive dictionaries. It would be better to have them be accessible as objects directly e.g. instead of:

from skplumber.primitives import classifiers
prim = classifiers["RandomForestClassifierPrimitive"]

It would be more natural to say:

from skplumber.primitives.classifiers import RandomForestClassifierPrimitive

Furthermore, it would be good to eliminate the Primitive postfix from all the primitive names; it's a little redundant.