Compute Froude and vmax only when needed

[lrntct]

• Resolves Velocity direction and Froude number should be computed on demand #7
• benchmark results for solve_h():
  • ~5.5% increase in compute time when those values continue to be computed at each time-step (unlikely to happen in real life)
  • 73% decrease in compute time when none are computed
• For the whole surface flow, this represents a ~30% decrease in compute time.

[lrntct]

The tests went from 6min to 5min 🙂