Use valid Standard Names for all external variables

[mdpiper]

The bmi-tester warns that four variables

1. precipitation_mass_flux_adjust_factor
2. precipitation_mass_flux
3. open_area_or_not
4. snow_class

in the ECSimpleSnow BMI are not valid CSDMS Standard Names. These variables should be updated to conform to CSN.

[wk1984]

I can not find the corresponding standard names for these parameters. So I keep them as now. Meanwhile, I think "precipitation_mass_flux" should be on the CSDMS standard name list.