reconcile initialisation off tss across apps

[JhanSrbinovsky]

from @har917 comment in #497. I had a quick look through the code again and I'm definitely suspicious about where/when %tss is being first evaluated (in offline and ESM1.6). It looks like this is in cable_soil_snow_main - CM2/AM3 has this in the _um_init section - but the variable is used in the albedo code, radiation and canopy sections.

I did come across some questionable examples of this.

Originally posted by @har917 in #497 (comment)