Add a new IO option for EnKF for FV3-LAM restart files in their original dynvars and tracer form.

[TingLei-NOAA]

In the current IO for Fv3-lam of the GSI EnKF, the read-in and write-output FV3-LAM restart files are combined into one single file from original UFS output's two restarts files of the dynvars and tracer files respectively. After EnKF update, those combined files would be "split" back to the original dynvars and tracer files. It was chosen to do so because to reduce the complicities in the EnKF IO coding.
Those combining and splitting are done using ncks now. Though those ncks process could be done in parallel, when the model domain become larger in a few major applications, removing those extra steps is regarded as needed to speed up those whole process.
Hence, this issue will document the effort to add an extra option for EnKF IO for fv3_lam (mainly on gridio_fv3reg.f90) to do reading/writing directly on the UFS original restart files of the form in dynvars and tracers files respectively.