Add some classes within Contract

Nothing is done with these classes yet
Add some more classes (draft)
Add more classes and start implementing Projection methods
Delete DataModifiedEnum and rename DataModified to Diff
Fill methods in Projection
Merge remote-tracking branch 'origin/master' into IsolateContractsAndProjections

Conflicts:
	EvernestFront/EvernestFront/EvernestFront.csproj
	EvernestFront/EvernestFront/SourceTable.cs
Remove logic from contract classes and replace some User methods

Move logic from contract classes to static classes Make...Contract
Fix spelling mistake and remove some heavy comments
Use more out arguments, replace some more User and Stream methods
Add User methods SetRights and IdentifyUser
Add Projection.TryGetUser. Add pull and push methods in User
Start refactoring unit tests
Merge remote-tracking branch 'origin/IsolateContractsAndProjections' into IsolateContractsAndProjections

Conflicts:
	EvernestFront/EvernestFront/Projection/Projection.cs
Continue refactoring unit tests

Everything except RelatedUesrs/Streams is done for User methods.
TODO : sources
Use SetItem instead of Add for dictionaries

Unit tests about User and Stream now pass
Add files for sources

Does not compile : answers files missing
Add missing answers and source tests
Isolate serializable part of events in a Contract
Fix .csproj
Add GetSource

Users and sources now only built through TryGetUser or TryGetSource. To
be done soon for streams.