rozpracovane mapped_data_collection
Merged with bugfix on data.dart
saving work
refactored collections merged in
test edited
Merge pull request #3 from hleumas/refactored-collections

Refactored collections
Merge branch 'master' of https://github.com/hleumas/clean_data into mapped-collection
Mapped collection almost done.
Working implementation of MappedDataCollection.