Fix reading of checkpoints with Parrinello-Rahman (modular simulator)

Fix reading of checkpoints with Parrinello-Rahman (modular simulator)

Using modular simulator, simulations using Parrinello-Rahman barostat
could not be read from checkpoint, throwing an error in the checkpoint
loading routine. While the legacy implementation of the P-R barostat
required the pressure at the previous step to be checkpointed, the
modular implementation does not require this. load_checkpoint is,
however, expecting this field to be present and throws an error.

This change fixes this by setting the globalState flags in dependence
of whether the modular simulator will be used, avoiding read_checkpoint
to expect this entry.

Note that tests ensuring this bug not to reappear are introduced in the
child change I3bcd0729.

Refs #3377 (fixes point 3)

Change-Id: If8afd294b8c79ceef66e71293d9d93cf2f7d0df8