Add bridges

[RegMeasures]

Add functionality to include the additional energy losses associated with flow impacting a bridge deck of defined elevation and location. Bridge deck changes flow by:

1. Additional energy losses
2. Reduced flow area (depending on deck thickness?)

May want to consider other types of hydraulic structure at the same time such as sluices/gates (energy losses?) or sub-grid scale bridge peirs/constrictions (reduced "effective area") of cells.

Looking at how other existing models handle these features would be a good starting point.