Allow for custom NWM cache dir

[SorooshMani-NOAA]

Right now it only allows True or False to be specified for cache argument of NWM config. This limits user to using home directory. On an HPC, a user might want to try and set a path on scratch to be the location where the NWM data is cached.

See:

CoupledModelDriver/coupledmodeldriver/configure/forcings/base.py

Line 833 in 7d2d85f

'cache': bool,

[SorooshMani-NOAA]

As noted in schism-dev/pyschism#123, adding custom path still doesn't solve the issue of need for internet access