Special characters in SRCROOT sometimes lead to build failures

In CESM, the CAM build fails if `SRCROOT` has a `+` in it. I don't know if other special characters also cause a problem, though I know `_` is okay. I think there are two paths forward, though maybe others can add to this list:

1. Determine what special characters are problematic, and then update `create_newcase` to produce a useful error message directing users to rename `SRCROOT` if any are present
2. Figure out what needs to be changed to allow these problematic characters in the directory name