Carbon: results summary and valuation

[dcdenu4]

Currently just interested in using social cost of carbon for valuation. Most interested in the change between scenarios, so we could just aggregate across the whole raster outputs or under Study Area parcels.

This issue breaking out from some previous discussion in #80

[davemfish]

On hold until we get some clear direction about what further valuation metrics are important.

[davemfish]

#120 will be useful for this

Oops, posted this comment on the wrong issue