Make local source term definition possible

Use the specified patch mesh instead of the entire bulk mesh to define
the source term.

MateriаlIDs based solid models in phasefield processes

Same changes as in PhaseField process.

The multiple inheritance for the phase field extension
is not actually needed and can be completely avoided.

Use Solid Material Models from MFront

... such that one can copy and paste the vtkdiff commandline to the
terminal and immediately run it

Tutorial books contain links to the old OGS-5 keyword documentation.

Implementation of HeatTransportBHE process

1) move function bodies from .h to .cpp file.
2) add documentation of the BHE_1U class.
3) add the default case for the switch.

Avoid duplicates.
Remove unused code parts.
Update source code docu.
Use STL algorithms.
Stronger assertions and improve error messages.
etc.