Rating curves / equations as downstream boundaries
At the moment we can use 'user control' links to act upstream of the DS boundary. These links have head/flow/power as their inputs. It would be useful to have a dedicated object that could be either a look up table of levels (rather than heads) and discharges, or an equation that would define the level / discharge relationship. Note that if an equation option were selected then it would need to be multi-part to allow for a changing relationship as water goes out of bank.