Add support for cpu only

[luhuhis]

#87 took the first step for this.

One way to incrementally add CPU support is to select a few executables and also try to get them to compile and run with the CPUONLY flag, which will then help other executables as well, as the code gets less and less hardcoded to the GPU backend.

The next step is then to make the CPUONLY flag an optional global CMake flag, which then disables FindCUDA etc and also the GPU backend entirely.

[luhuhis]

In #95 the next steps have been taken