Should we maintain SCREAM_HACK_XML to allow manual edits to namelist_defaults.xml

Currently, if you do `./xmlchange SCREAM_HACK_XML=TRUE`, you can then manually edit the namelist_defaults.xml file, and be sure that buildnml won't overwrite it (in particular, any atmchange call will have no effect).

Should we maintain this? Does anyone think it will be important to have it? I ask because every feature has to be maintained, which can be a burden as things are updated/upgraded/fixed. My last round of upgrades/fixes to atmchange/buildnml made me realize that we may be ready to drop the hack capability. We went through some iterations of our buildnml stack, and I think we now support a fair amount of capabilities. I don't see why one would want to hack the XML file manually.

@AaronDonahue @PeterCaldwell @ndkeen @crterai @jgfouca pinging you all for an opinion, since you are big users (or devs) of the atm configuration system. As things stand, unless I don't see any opposition, I'm going to prune this capability in the next few weeks.