Remove confusing double definitions of soil pools

[ccarouge]

CASA defines 3 soil pools, but there are variables in CABLE (ncs and nsCs) that declare 2 soil pools. This seems to come from a difference between the old bgc% carbon stuff and CASA.

It would be good to review and see if:

• we could remove bgc%.
• OR we could expand bgc% to use 3 carbon pools (even if one is empty)
• OR we can use comments and clear variable names that show these are for different algorithms, clarify why there is ncs and nsCs that seem to do the same thing
• OR with splitting cable_define_types.F90 into submodules per type, ncs could be defined only for the bgc module, it would bring some clarity.

In short, we need to check how these variables are used in the code, whether we need to keep the bgc% stuff or not and deal with it as appropriate to avoid confusion on number of soil pools.

See discussion #630 (comment)