[common] unify BaseExtractor and BaseVideoExtractor pipelines

[fabiocarrara]

visione/visione/services/common/extractor.py

Line 13 in 36cf0db

class BaseExtractor(object):

visione/visione/services/common/extractor.py

Line 143 in 36cf0db

class BaseVideoExtractor(object):

These two classes that orchestrate feature extraction from frames and videos share most of the functionalities, and the former can be generalized in the latter.