Move GIUH and Nash-Cascade to external

[SnowHydrology]

CFE uses the geomorphic instantaneous unit hydrograph (GIUH) and Nash-Cascade model to route surface and subsurface runoff, respectively. Other models implemented for NextGen will likely use these routines as well.

Current behavior

GIUH and Nash-Cascade code are part of the CFE repo and will have to be ported to other models.

Expected behavior

GIUH and Nash-Cascade code should be moved to their own standalone repos or a NextGen utilities repo. This way each model formulation that requires them, including CFE, can import the code as a GitHub external submodule. This will prevent us from having to maintain duplicate code in multiple locations. (To note: we have fixed this same-code-multiple-locations issue with AORC forcing and PET recently.)