Decouple coordinates buffer management from buffer ops in NBNXM

Decouple coordinates buffer management from buffer ops in NBNXM

To make transition to the new device-side state propagator data
manager easier, the copy and plain buffers management have to be
decoupled from the actual operations on data in the buffers.
This commit splits the corresponding functions into two, and
decouples CPU- and GPU-based coordinate buffer ops one from
another. Since the coordinates are now clearly communicated
between PME and NBNXM, the plain pointer on the CPU-side code is
replaced with the DeviceBuffer<float>. The getters for the device-
side buffer now never return nullptr, but exit with the assertion
if the corresponding data is not intialized.

Change-Id: Ic793f815870a8b4f414a9b7ca0a5001e58f49c7a