Make PME GPU structures use DeviceBuffer

Make PME GPU structures use DeviceBuffer

A couple related function calls in PME are changed to match.
allocateDeviceBuffer() is now correctly called with the GPU
context handle instead of a stream handle (this was not causing
errors since context is only a stub in CUDA implementation).
pme-gpu-internal.cpp is now compiled with CUDA compiler
(to handle gputraits.cuh included through devicebuffer.h),
paving the way for using unified CUDA/OpenCL host code.

Change-Id: I3621bd93e4b2cc7c02161323f532ca62b48763ab