PME spline+spread CUDA kernel and unit tests

PME spline+spread CUDA kernel and unit tests

The CUDA implementation of PME spline computation and charge spreading
for PME order 4 is added in pme-spread.cu.

The unit tests for PME CPU spline/spread stages
(e8cf7c0) are also extended to work with
the PME CUDA kernel, using the same reference data.
The tests iterate over all CUDA GPUs which are compatible with Gromacs.

Refs #2054, #2092.

Change-Id: If5ec49f030b9b94395db28fa454ea25c3efb05d1