How to handle modified CESM code required by DART

[kdraeder]

Use case

Assimilate using one or more CESM component models and DART.

Is your feature request related to a problem?

That requires modification of a few CESM fortran modules, scripts, and resource files.
The modified set changes with each version of CESM which is interfaced to DART.
The modifications are developed by specialists in each component+DART and generally not combined into a comprehensive set.
Making the modifications available to users is currently awkward; they are not part of the DART repository, so they must be fetched from somewhere else.
The versions of CESM which can be used with DART are limited, and typically lag the released versions by at least several months, and lag the development versions by (much) longer, which reduces the usefulness of DART.
These modifcations may become more difficult in the age of NUOPC in CESM.

Describe the your prefered solution

Integrate DART into the development and testing process of CESM, so that these external modifications are not needed and DART works with every released (tagged?) version of CESM. I came close to doing that for CAM in 2018-19, but reached an impasse about an inconsistency between the contents and file names of some history files. That (sort of thing) may still prevent the best solution.

Describe any alternatives you have considered

If that's not possible (or worth the effort) then some improved way of handling the required modifications could be worthwhile. Some factors to consider;

1. each component interface has been developed for the version(s) of CESM which made sense at the time,
2. some of the modifications can be handled cleanly through CESM's SourceMods mechanism, but some can't,
3. we'd like it to be easy for users to get and install the right modifications for their case,
4. they should be version controlled.