fix topsoil function

[paolap]

The current topsoil function is incorrect and at the best the one defined in APP4 will only suits specific soil depth levels.
The function should return the values for the top 10 cm of soil. So depending on the depth_bnds we should be able to interpolate this value.
AUS2200 top level is exactly 10cm so it cannot be used as an example

[paolap]

I guess we could loop on the maximum boundary of depth_bnds, if that's less than 10cm than you can accumulate the all layer, if it's more than you calculate a proportion of what's in the first 10cm respect to the total thickness

[paolap]

of course cable doesn't provide the depth_bnds base don this: ACCESS-Community-Hub/APP4#13

[paolap]

Need to double check this is correct but I added function

[paolap]

still need to update mappings and this should be also used for tsl and any other variable which is defined on "sdepth1"

[paolap]

mappings updated