Move settings from use_cases to namelist_defaults

Let me preface this by saying that I've never fully grasped the motivation for having use_cases (`bld/namelist_files/use_cases`), so I may be missing something here. But that said:

I'd like to see use cases removed and replaced with their analogous settings in the namelist_defaults xml file. My rationale is: even though use cases make sense for what they do, it's confusing and error-prone to have this additional mechanism that is just used for a handful of namelist variables. Even if the organization of namelist_defaults is less ideal for these variables, I think the upside of consistency and only needing to look in one place outweighs this downside.

I think this is actually a bigger problem because of how few variables are set via use cases. I'd say that 99% (or more) of the time when I'm messing with namelist defaults, I don't need to think about these use cases... which means that the 1% that I *do* need to look there, I forget to do so.

Definition of done:
- [ ] Simplify the use-cases to remove unneeded variables
- [ ] Have use-cases only have sim_year, sim_year_range and rcp in the use cases, and move all of the other variables that are currently in use cases to namelist_defaults (with dependence on those three).
- [ ] Move use-cases as a setting in namelist definition, and move the settings into the namelist_default file using use_case as the attribute that distinguishes the different options
- [ ] List of things in discussion below...