[FEATURE REQUEST] Give HEMCO the capability to read/write data as REAL*8

[yantosca]

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

Long story short: It is desirable to extend the HEMCO's internal data structures so that they can carry either REAL*4 or REAL*8 data arrays. Currently, HEMCO's data structures allow REAL*8 data to be read in from disk, but not to be written out to disk. HEMCO can only write REAL*4 data to disk.

Recent work by the GEOS-Chem Support Team has shown that writing restart file output as REAL*8 removes the small numerical differences seen when comparing GEOS-Chem "Classic" simulations that are run as a single stage (e.g. yearly) as opposed to simulations that are broken up into smaller stages (e.g. monthly) and are reinitialized from the previous stage's end-of-run restart file.

[yantosca]

I just realized this is a duplicate of #18. I'll close this one now.

[liuyuyu223]

May i ask whether there are difference if i only want to HEMCO the capability to read/write data as REAL*8? I ask because i found the code in
geoschem/geos-chem@652bb23 are mainly for version 13, while i am using version 12.5?