simplified the kernel factory infrastructure by omit the enum (Kerneltype) -> adding new kernels to the factory should be straightforward now

Remove VirtualFluidsDefinitions.h header file.
Remove redundant compiler definitions from gpu.cmake.

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

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

Remove unused stuff and the global include from VirtualFluidsDefinition.h. Adds missing export includes.

Warning: [-Wbackslash-newline-escape]
https://stackoverflow.com/questions/31673945/clang-backslash-and-newline-separated-by-space-in-single-line-comment

fixes some bugs for new updateGrid27 function, especially initialization of boundary condition coutners