calc_prod_impacts: Calculate productivity impacts
In TII-STP/pagglomR: Calculating the Agglomeration Impacts of Transport Projects

This function reads in Do Minimum and Do Something effective density matrices, and jobs data and applies the TII PAG Irish agglomeration parameters to calculate productivity impacts for the relevant modelled year.

1	calc_prod_impacts(eff_dens_dm, eff_dens_ds, jobs)

`eff_dens_dm`	Do Minimum effective density object create by `calc_eff_dens`
`eff_dens_ds`	Do Something effective density object create by `calc_eff_dens`
`jobs`	a dataframe of jobs in each sector for each modelled zone. If the analysis is being undertaken using the TII National Transport Model, the `ntpm_jobs` data file provided with the 'pagglomR' package can be used. For other models, this file needs to be prepared for the transport zone system in question.