OrificePrepareForGate Method
Set orifice discharge for al positions of the gate. If a control is active (a weir or gate is present)
then this must be called after:
1) Prepare()
2) Gate polygons have been defined
3) MaxGateLevel and MinGateLevel have been set.
This is called automatically in prepare. It must be called explicitly if
gate polygons or max/min gate level are changed after the prepare step