nit:  I know GFS uses 'ens' but I think its probably better form to internally use readable names and convert GFS to match conventions.

n.b. If we really wanted to be proper we would use [CF convention names](http://cfconventions.org/standard-names-26.html)(not sure why GFS doesn't?) which would mean calling an ensemble a 'realization', but thats probably more work than its worth at this point.