Special characters in SRCROOT sometimes lead to build failures

[mnlevy1981]

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

[github-actions]

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