Add utility to recenter snow ensemble increments

[CoryMartin-NOAA]

We are going to try to just recenter the snow ensemble to the deterministic snow analysis after a conversation with myself, @ClaraDraper-NOAA and @dtkleist .

First, an OOPS-based application will be created in GDASApp that will:

• compute the ensemble mean forecast of the snow variables
• subtract the ensemble mean from the deterministic snow forecast
• add the increment and the above difference together

Then the recentered increment will be applied to every ensemble member so that their spread is identical between forecast and analysis.

fregrid will be used to interpolate between the deterministic and ensemble resolutions At first, only the above will be an OOPS-based application, eventually we may wish to OOPSify the code that applies the increment to the forecast for the snow analysis.

[CoryMartin-NOAA]

Work will be done in feature/snow-recenter