Reorganize on-GPU PME force reduction flag handling

Reorganize on-GPU PME force reduction flag handling

Instead of passing around a flag everywhere that tells PME whether
forces are reduced on GPU or CPU (and whether transfer needs to happen
for the latter), we pass the flag once when configuring PME for
the next step and store it internally.

Refs #3029

Change-Id: I81fa2dc93dd979e2b85b4d7fe8cf266a3fde9b8f