add used backend to compiler flags

- GOOFIT_THRUST_BACKEND_OPENMP for OpenMP
- GOOFIT_THRUST_BACKEND_CUDA for CUDA
remove unused functions
properly set MaxFunctionCalls/MaxIterations after adding the parameters (based on NFree)

- parameters now identical to RooMinuit when used with strategy 2 and minuit 2
use backend #define instead of "OMP_ON" in GlobalCudaDefines.hh