Add Hazen-Williams and Darcy-Weisbach friction formulations

[mdbartos]

Add HW and DW friction formulations to enable easier conversion between EPANET and pipedream models: https://epanet22.readthedocs.io/en/latest/3_network_model.html#id2

Essentially this involves creating an alternative formulation for S_f in the momentum equation.

Ideally, this formulation should work for open channel flow as well (make sure pipe diameter D can be replaced by hydraulic radius R).

[mdbartos]

@minsky97: Can you look into this issue?