Relax water level requirements when opening a lock in phase wise operation

[erwin-mulder]

In phase wise operation with D-Flow FM, the head level may have been changed relative to the lock after 'leveling' in a previous timestep. This causes an assert to fail and the calculation not to continue.

TODO:

• Remove the assert (and thus allow the levels to differ)
• Introduce a a warning if there is a difference, larger than a parameter (to be introduced).
• For operation in D-Flow FM, make the allowed difference configurable in the .ini file.
• For operation in D-Flow FM, the default allowed difference should be 10 cm (if not configured)