calc_eff_dens: Calculate effective densities
In TII-STP/pagglomR: Calculating the Agglomeration Impacts of Transport Projects

This function reads in a list of generalised costs by origin-destination pair and also a dataframe of jobs by sector. It calculates the effective densities for each modelled zone and for each employment sector.

1	calc_eff_dens(gen_costs, jobs, year)

`gen_costs`	a dataframe of generalised costs in the following format: 'origin_zone', 'destination_zone', 'generalised_cost'
`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.
`year`	the modelled year. It is important that this is correctly specified for the purposes of the productivity calculations and subsequent discounting required.