Add an exchange module for the MCT

[AntoniaBerger]

Fixes #252

Work structure:

• Create a new exchange modul. #256
  • New Base class 'IPhaseTransition Model'
  • New class 'LinearExchange'
  • Add to CMakeList
  • Add the linear exchange to the MCT model. #262
  • Compare exchange modul results with implemented version.
• Add exchange as a part of the MCT modul
  • make exchange parameter parameter part of MCT
  • ubdate parameter handler for phaseTransitionModel
  • Test results
• Outsourse Iteration and declaration for residual and Jacobian?

[jbreue16]

I had a couple of minor remarks that need to be addressed before merging.

I thought about adding a dummy Exchange (similar to what we have for binding), but decided against it since there should always be some exchange in the MCT, as using that model doesnt make sense without.

Tests still work, so I'll approve this PR to be merged after minor revision.
It would still be interesting if the abstraction layer has any influence on the performance of the MCT. A benchmark would be nice, comparing the old and new implementation wrt computational performance.