Updated file format for Jails

[colincadams]

Is your feature request related to a problem? Please describe.

While working through some open questions on the PRD, it looks like there are some minor changes that we should make to the file formats to make everything work as expected.

Describe the solution you'd like

At a high level the changes are:

• percentage covered county and percentage covered population will now be columns instead of their own metrics
  • this makes more sense because now it is clear that they will be different per metric, the incarceration rate metric will be able to include a superset of the counties that are included in jail population in some cases. The coverage for the incarceration rate metric is what should be used for the key insights block
• community population will now be included in the county name file, instead of as its own metric
  • while we will use the population over time for calculating the incarceration rate, the frontend only needs the current value and doesn't display this as a metric over time at all -- it is just context about the county so makes more sense to live in that file.

I will update the fixture file and docs and link them here, but at a high level what this means is that there will only be two metrics in the data file (POPULATION_JAIL and INCARCERATION_RATE_JAIL).

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context about the feature request here.

[colincadams]

See https://docs.google.com/document/d/14RZM1KchEnMrc19vpr_kKxWE3bY2BhGzoMEP37Fra30/edit#heading=h.6gz7jlt22qlp for a description of the updated format. The existing fixture files in Google Drive have been updated, and are linked to from that doc.

[colincadams]

Updated docs and files to add the PERCENTAGE_COVERED_COUNTY metric back, as @nasaownsky pointed out that it will simplify the frontend implementation.