Add sfc increment ensemble mean calculation

[ClaraDraper-NOAA]

The new soil analysis requires adding the ensemble mean increment to the deterministic member. For the atmosphere the same code is used to calculate the ensemble mean states and ensemble mean increments: gsi_utils.fd/src/EnKF/gfs/src/getsigensmeanp_smooth.fdgetsigensmeanp_smooth

For the sfc files, this code calculates the ensemble mean states:
gsi_utils.fd/src/EnKF/gfs/src/getsfcensmeanp.fd/

We need to

1. update getsfcensmeanp.fd to also handle sfc increment files, following the atmospheric example.
2. getsfcensmeanp.fd assumes the Noah model variables, and needs to be updated for Noah-MP. For netCDF files, this update is very minor.