Add L2SM model into JULES

Request to add the L2SM model into JULES as an optional replacement for the current Sellers scheme. Ideally the new branch should be submitted to the trunk.

The master version of L2SM is here:

but this is a very generalised form of the model, written in Python, and we (probably Tristan) would need to boil it down into a single Fortran function.

Note - a key task will be deciding how we input vertically varying canopy properties to JULES. Probably, in the first instance, this should be done on a per-PFT basis in the appropriate NML file.

Thanks, Tristan:
I will get working on this.

Hi Tristan:
Just a quick check-in.
I was able to fork your layeredCanopyTwoStream repository, then run both and with python2 on JASMIN. The first program produced numbers and the second program produced plots, both without error.
The checked-out code is now in:

I guess you want me to translate the function get_layered_canopy_fluxes( ) into FORTRAN. Let me know if this is not the case. Otherwise, I will try to do that next week.