Provide calculations for values that appear in outputs report

[matt-dray]

The outputs report recreates charts and tables from the outputs app. In the underlying code we also generate values that are used in the body of the report that don't exist in the outputs app.

In the first instance, we could add instructions to project information so schemes can calculate these themselves from the Excel file downloaded from the outputs app.

In theory we could offer these calculations as part of the outputs app itself, but it could be fiddly (e.g. it may require some backporting, dependent on the model version that was used for a given scenario).

FYI @jacgrout.