Review cable_runtime%coupled_run switch

[JhanSrbinovsky]

          These have been here since ACCESS-1.3 days and we just keep adding switches. The canopy% fields (like Claire has been looking at int JAC version of climate% decs) are all zeroed at declaration,allocation as was required there. If not zeroing for the moment is not causing a problem then I suggest we wait until we pick this up and be sure to CALL the zeroing SUBR.  AND remove this code. Note aswell the coupled_runtime flag has effectively been re-purposed and should at least be renamed to reflect that it does not indicate whether it is in a coupled run or not. I'll make an issue from here

Originally posted by @JhanSrbinovsky in #527 (comment)