Catch errors in v-rescale and Parrinello-Rahman checkpointing
This adds additional checks to catch checkpoints missing the v-rescale thermostat integral or the Parrinello-Rahman box velocities. This also make the number of steps used in the exact continuation tests explicit, which will be useful in further investigations of #3518 (closed).
In partial fulfillment of #3518 (closed)
Edited by Pascal Merz