Add number of convictions filed to analysis

[ghing]

@GovInTrenches writes:

Actually, we do know how many cases were filed in the time period.

https://performance.cookcountyil.gov/Public-Safety/Number-Of-Felony-Cases-Filed-By-Felony-Class/kcfs-dufb

https://performance.cookcountyil.gov/dataset/Quarterly-Data/wvz2-c4cu?q=misdemeanor

See also: https://datacatalog.cookcountyil.gov/Public-Safety/Public-Safety-Q3-Performance-Management-Goal-Asses/iwtc-d53w
Let’s compare these numbers as part of our analysis.

[ghing]

This should have gone in the cook-convictions project. Closing this one in favor of sc3/cook-convictions#144