[FEATURE REQUEST] Allow for flexible precision I/O

[msulprizio]

To reduce differences between single and multi-segmented GEOS-Chem simulations, we need to be able to save out and read in restart fields at REAL8. Right now, HEMCO is hardcoded to use REAL4 for data I/O.

[yantosca]

This is related to issue geoschem/geos-chem#111.

[lizziel]

I am going to take a look at this and see if I can update HEMCO to read and store R8 as well as R4. It looks like we need this capability for CESM. It would also be good to finally get bitwise reproducibility when running GC-Classic.

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