Get composite version of DICE working

[corakingdon]

Branch composite on MimiDICE2010 here: https://github.com/anthofflab/MimiDICE2010.jl/tree/composite

[corakingdon]

Try

pkg> add MimiDICE2010#composite

m = MimiDICE2010.get_model_composite()
run(m)

[corakingdon]

This works now, but I want to wait until there are better datum access methods for composites and then add a validation test file for this composite version. We should also decide what we want our convention to be for model packages going forward: do we want get_model to return the composite version or the flat version? Do we want a keyword argument for which kind it should return, or a separate function?

[corakingdon]

update: Lisa has added validation testing to the composite version, now we just need to decide what we want our convention to be for a "get_model_composite" function, or if we should release new versions of the model packages where the composite version is the main version