Implements pressure and stress correction for multiphase model with density contrast. Pressure is regularized and hence different from the form used by Safari and Fakahri. Surface tension is not implemented.

Implements cumulant phase field function and a passive incompressible momentum equation. The phase field is only coupled to the fluid velocity but there is no featback to the fluid. Only as a starting step for further developments.

Fix various msvc compiler warnings. With msvc the compiler flags are explicit passed to nvcc with -Xcompiler.

 - metis: do not pass -O3 flag on msvc compiler, as this flag is ignored anyway. Instead of /w pass w0, which is equivalent to /w.
 - RheologyK17LBMKernel: A and B are also defined in D3Q27System.h. Therefore to not hide this declaration they are renamed to A_ and B_
 - additional small clean ups...

Remove debugging ls. Activate double accuracy for the numerical tests. Deactivate 256 & 512 grid sizes.

Add gpu options to root cmake. Add the cmake option for passing the path to the numerial tests. Add git clone test to the CI.