Make carrier specification comply with javascript frontend supported styles

[vid8687]

Specifying multiple carriers as a single string is currently not supported; knit(..., "CA CB") will fail even if CA and CB have been specified in the initial carrier list. This is probably slightly different from the options the js frontend allows.

[ixchow]

Opinion: My feeling on this is that specifying a list as a space-separated string is a bridge too far even for dynamically-typed languages. (In general, I'd prefer if we chose one right way to specify things instead of trying to support all possible ways.)

[vid8687]

I am totally okay with that, I think then the js-frontend shouldn't support the multi-carrier strings which it seems to. (They should probably try to be as similar as possible in formats they support although I suspect only a few people would switch between the two frontends)