RuSentRelOpinionCollectionFormatter -- move all the opinion iteration during saving/loading into base class

[nicolay-r]

Logic become compex and hence some general aspects considered to be declared in base class.
Since formatter could save/load all the opinions or select only those which labels could be formatted, there is a need to perform the related refactoring

AREkit/contrib/source/rusentrel/opinions/formatter.py

Line 10 in fefe27d

class RuSentRelOpinionCollectionFormatter(OpinionCollectionsFormatter):

This issue proceeds the #139 and the related bug #137

[nicolay-r]

Splited onto provider and writer parts, where provider also depends on the opinion converter (string -> Opinion type). See #210