hist_str list

[bitterbark]

I have a PR for the ADF that implements hist_str as a list and I'd be willing to get that into the CUPiD version if it looks compatible and if someone else doesn't want to/hasn't already done it.

(from @mnlevy1981 , "MOM6 splits variables across a larger number of history files than other components, so at some point we may need to support hist_str being a list (which probably requires renaming it, since it's no longer required to be a str)."