CSIT-1110: Prepare for migrating the new detection

+ Do not declare BitCountingClassifier.classify() as class method.
+ Make BitCountingGroupList subclass of list.
+ Inherit from abstract classes whenever possible.
+ Drop unneeded imports.
+ Add module docstrings and class docstrings anywhere.
+ Add TODOs hinting at possible improvements.

Change-Id: Iccfff5c0e7be0607d6cfa74314083fcfe5a4d7d9
Signed-off-by: Vratko Polak <vrpolak@cisco.com>