Cleanup unused code
Added wrappers to check for errors in CUDA api and kernel calls
Added the api call check to fields.cu; works for e.g. malloc larger than device memory, or free of invalid device pointer
Added kernel check as a test in fields.cu, works when e.g. calling a cuda kernel with host pointer
Initialize GPU pointers at zero
Implemented the 4th order RK4 including the offsets
Merge remote-tracking branch 'chiel/cuda' into cuda
Fixed ugly bug in advec_4m