Step Export - Graph optimization (decrease the file size) #190

Step Export - Graph optimization (decrease the file size) #190

Functionality to remove duplicate entities from Step graph is added.
Class MergeSTEPEntities_Merger is main entry point.
Class MergeSTEPEntities_EntityProcessor implements the basic replacement
logic.
Children of MergeSTEPEntities_EntityProcessor implement the logic for
the replacement of particular step entity.

Google tests are added for new classes.