Set up workload data structures

Set up workload data structures

This change estabilishes three data structures for describing the
computational workload within a run. Flags related to the workload are
categorized in three groups based on their lifetime:
- per-step flags in the StepWorkload class
- domain liftime / nslist steps in DomainLifetimeWorkload
- constant over the entire simulation in SimulationWorkload

The present change only introduces naming by: renaming of the
former ForceFlags to StepWorkload and PpForceWorkload to
DomainLifetimeWorkload.

Moving flags from e.g. the force schedule into these will follow.

Change-Id: I2bcc911091e2dd8ca0bcbf53a40dfcda09ba368b