Rename conversion function
Refactor gpu_utils

Moved gpu_utils.h to gmxlib/gpu_utils.h and named most of its
functions in a GPU-flavour neutral manner. Renamed gpu_utils.cpp =>
gpu_utils_ocl.cpp so that we can have a non-GPU gpu_utils.cpp to
implement the setting of NBNXN allocators outside forcerec.cpp.

Moved pmalloc_cuda.h to gmxlib/cuda_tools. Likewise, moved the OpenCL
alloc and free to gmxlib/ocl_tools, but didn't give them their own
file, since the original reason for the separate pmalloc_cuda.h file
was probably just to facilitate the explicit mention in forcerec.cpp
that is removed here. Not sure what we should do in the long term
here.
Merge pull request #84 from mabraham/opencl

More refactoring to reduce duplication