Further refactor IO

[charleskawczynski]

This PR makes some of the IO functions, e.g., write_field and add_write_field etc. accept group, instead of a NetCDFIO_Stats, and the group is now extracted outside. This makes these io function more low-level and alleviates our dependence on NetCDFIO_Stats. I think that we can apply this to other functions, but I'd like to do this incrementally.

[ilopezgp]

bors try

[bors[bot]]

try

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