DeviceBuffer headers are added

DeviceBuffer headers are added

For now they contain templated versions of GPU memory (re-)allocation
and host-to-device copy routines. The DeviceBuffer declarations
and the freeDeviceBuffer() implementations are moved there as well.
Using those, the GPU pairlist allocation code is made look
mostly the same in CUDA and OpenCL. ocl_realloc_buffered() is
removed; cu_realloc_buffered() is to go next.

Change-Id: Ic724113d7d0ed6b4707d094010e11ccbcd5da3c5