moved To... extension methods to be more consistent in where they can be found

[Kasdejong]

refactored extension methods around the elementmodel to now be in the appropriate classes

resolves #2992

[ewoutkramer]

And add the breaking changes to the wiki! These will actually cause pain since if you used the SDK to work with multiple versions of FHIR in one solution, you will have used the names of the Extension classes, so its worth documenting.....