improved test or example case for ngen/snow17

[andywood]

The current first example case for snow17 uses cat-27, which pre-existed in the ngen data examples. It does not have low enough temperatures to simulate snow.

Two additional cases are suggested:

• a HF catchment (or camels basins) where snow is part of the hydrologic cycle
• a point snotel station simulation so that the outputs can be compared to obs

A full snotel network simulation could also be set up for nextgen evaluation across different model formulations.