- split data_entities from entity_collection in 2 diff modules
- renamed data_entities to data_entity, also the test modules for data_entities
- added more unittests to data_entities and to entity_collection
- fixed a series of bugs in data_entities
- added docstrings to some methods
- added remove_all and get_all to the collection class
- fixed some bugs in EntityCollection. replace - added some corner cases
- refactored get_by_* methods
- added some more methods on data entities regarding identifiers