Enable writing of coupler auxiliary history files for 6 hour forecasts

[kdraeder]

@jedwards4b ,
2 modules that are involved with writing coupler auxiliary history files
assume that forecasts will be at least 24 hours, andor will end on 0 UTC.
I have a partial fix for that; it handles 6 hour forecasts, and just for the a2x files.
The partial fix looks straightforward to install in cpl7.0.14-2-ga4b9e6e:
components/cpl7/driver/main/{cime_comp_mod.F90,seq_hist_mod.F90}

I don't know whether a more general fix is justified at this time.
WACCM(-X)+DART does use a shorter forecast (and future high-topped models will),
and there are plans to do assimilations from which these files might be written
at different frequencies, so a generalization of this fix may be worthwhile.

A second question is whether to upgrade maint-5.6 first (where the fix was developed)
or to proceed to the end goal; upgrading a recent version of CESM which will be carried forward.

[jedwards4b]

You will need to update maint-5.6 for use of cesm2.1.x.
Can you test with cesm2.3 and determine if this is still a problem?

[kdraeder]

I can do a test in cesm2.3, but the relevant sections of code haven't changed in meaningful ways
since maint-5.6, so I'm sure that it's still a problem.
Would it be useful to have a test case to look at before installing a fix?

I'll update maint-5.6, for completeness, and it might be useful.

[jedwards4b]

The mct coupler has been deprecated and will soon be removed. New testing should be done with the cmeps coupler which includes a complete rewrite of
coupler IO.

[kdraeder]

It turns out I won't be able to test this until (url)(https://github.com/ESCOMP/CAM/issues/790) is fixed.
I don't know the schedule for that, but it seems to be soon.

[kdraeder]

Issue 790 is fixed, and I've pulled all the related fixes into a CESM2.3.
My test there shows that "daily" aux cpl hist files are not written at the end of a 6 hour forecast.
The hourly and 3-hourly files are.
Since the fix I have is limited (only for a2x files and 6 hour forecasts),
should I include it here for discussion, or open a PR?

[jedwards4b]

It's easier to discuss if you open a PR. Thanks

[kdraeder]

This appears to be resolved in CESM2.3 (CMEPS) by CMEPS PRs 379 and 378.
Thanks for all the recoding!