rozpracovane mapped_data_collection
Merged with bugfix on data.dart
saving work
refactored collections merged in
test edited
Merge branch 'master' of https://github.com/hleumas/clean_data into mapped-collection
Mapped collection almost done.
Working implementation of MappedDataCollection.
Minor refactoring and documenting.
Sorted collection view almost done/
Almost done.
Indexes + transformed collections + test suite
prettify() removed from data._notify()  sync
excepted collection refactored bug fixed
Bugfixes in change set
Refactor filtered collections.
Remove unused SetOp2 class.
Remove whitespace.
Remove unnecessary clear.
Simplify mapped collection.