Fluid: Initial changes for OpenMP GPU support
Contains basic support for OpenMP GPU offloading.
That is, offloading of fluid KERNEL loops to the GPU.
This branch offloads pressure and advection calls only - the 2 most
expensive operation per step. In theory though, any function can be
For now, this branch needs to be build with a compiler that supports
Nvidia GPU offloading. Exact GPU models need to be specified via CMake.