corrects pressure update in MultiphaseTwoPhaseFieldsPressureFilterLBMKernel, MultiphasePressureFilterCompressibleAirLBMKernel and MultiphasePressureFilterCompressibleAirLBMKernel. MultiphasePressureFilterCompressibleAirLBMKernel is not yet different from MultiphasePressureFilterCompressibleAirLBMKernel and hence not yet compressible.

add pressure filter kernel with one phase field / add double ghost layer connectors for two distributions and pressure

fix bug and remove redundant initialisation in MultiphaseSetKernelBlockVisitor / remove unused parameter in MultiphaseInitDistributionsBlockVisitor

Velocity fomulation without viscosity correction and no output for pressure and wrong output for velocity

Update cmake to dev version.

Move VirtualFluids header to src.
Add CI file and add files, only existing in the open source version.

Therefore the muparser cmakeLists.txt was used, but a modified:
 - name is corresponding to the original file: muparser (in lowercases)
 - removed the install
 - adds public include directory : CMAKE_CURRENT_SOURCE_DIR/include.  --> also adapt the include from the dependent fles.
 - adds link to OpemMP

CXX 11 is now required. (CMake throws an error if cpp11 is not available).

Make the cmake project more flexible while removing the CMAKE_SOURCE_DIR usage.