Add an untested_use_anyway namelist option

Migration of trello issue.

There are a number of aspects of CLM that we don't test. Mariana suggested having a namelist option like untested_use_anyway. So the run will die if you try to turn on an untested feature, unless you also set that namelist option.

Then incrementally add things that trigger that.

Name the variable: use_untested_experimental, use_untested_science, or use_experimental_science