#1448 Gave some nice performance improvements. With these changes, a small number of utility functions are now dominating the profile (for the RWS HWS model):

• set_flow! because of a dictionary lookup. To resolve this without much added complexity, I propose to not (just) store inflow and outflow IDs like in Pre-calculate flow neighbor IDs for Pump, Outlet, UserDemand, FractionalFlow, TabulatedRatingCurve #1448, but to store the EdgeMetadata of the inflow and outflow edges (which also includes the inflow and outflow IDs). Also add the flow index to the metadata (and make the distinction between that and the edge ID clear), so that it doesn't have to be retrieved by the aforementioned lookup.
• get_level because of a basin index lookup. Per node type whose formulate_flow! uses this function:
  • PidControl will probably never be used heavily enough so that this becomes a performance bottleneck
  • UserDemand: store the upstream basin index per UserDemand node
  • ManningResistance: store the upstream and downstream basin index per Manningresistance node
  • LinearResistance, Outlet: store the upstream and downstream basin index per LinearResistance node (although both can be LevelBoundary nodes, not sure yet about this)
• basin_bottom also because of a basin index lookup. This function is only used for TabulatedRatingCurve, so I propose storing upstream and downstream bottoms in the TabulatedRatingCurve struct.