splitting args for carriers to allow plating

[heread]

Kausi added a line to split the args of the carriers (so eg ['6, 4']) into a list of strings (['6', '4']), so that plated carriers don't trigger the value error below. This allows the python frontend to generate knitout code with plating

[ixchow]

Looks reasonable to me. (Though, relative to your example, it would probably split '6, 4' into ['6,', '4'].) Probably impacts discussion in issue #2, but does not resolve it.