StatePropagatorDataGpu object to manage GPU forces, positions and velocities buffers

StatePropagatorDataGpu object to manage GPU forces, positions and velocities buffers

In current version the positions and forces on the GPU are managed by different
modules, depending of the offload scenario for a particular run. This makes
management of the buffers complicated and fragile. This commit adds the object
responsible for management of the GPU buffers of coordinates, forces and
velocities. The object is connected to all clients that use coordinates, forces
and velocities buffers, while keeping the existing logic intact where its
possible.

Since the H2D and D2H copies are now done in nullptr stream, some of implicit
synchronization is lost. Consequently this commit does not always work
properly with newly introduced buffer ops / halo exchange features. To avoid
the confusion, GPU buffer ops are disabled by the assertion. There will be
a separate commit with all copies done synchronously, which will work
with the buffer ops. The stream- and event-based synchronization will be
introduced in the follow-up commits.

Refs. #2816.

Change-Id: I2e2ba1b6436f087d1f2fef4ff876445814a724e7