Fix nc2npz.py for processing constants fields.

[arthurfeeney]

There seemed to be two issues with nc2npz.py

1. The constants seem to be downloaded into constants/constants.nc, rather than just ./constants.nc, so they're in a subdirectory.
2. The CONSTANTS list was not using the nc variables names. It looks like CONSTANTS is used elsewhere, so I added CONSTANT_VARS, which uses the nc variable names.

I think this fixes my issue #125.