Fix appeals vignette EAV aggregation method

[kyrasturgill]

While reviewing possible reasons for Appeals vignette agency totals issue #46 , an error was identified in the way the appeals vignette adds pre-appeal EAV back into the base for taxing districts. Currently the code aggregates the EAV difference between stages at the tax code level, while it should aggregate up to the agency. This results in agencies that cover multiple tax codes to have their agency rates vary across tax codes, while agencies should have a uniform tax rate regardless of the tax code.

This PR adjusts the code to ensure the EAV difference within each tax code is summed across all tax codes of an agency. This fix does help in aligning the summed amount taxed by agencies between stages, which was the issue identified in issue #46 (this flagged that taxes collected by Cook County in 2021 would be 16% higher in the mailed stage than the board stage, when they both should theoretically remain close to that agency's extension). With this adjustment, the entirety of taxes paid Cook County for the mailed and board stages vary by 3.8% rather than 16%.

[kyrasturgill]

This change in methodology results in greater differences between median residential tax bills - median residential tax bill in 2019 certified stage increases by 11% rather 7%. This has been adjusted in the text of the vignette.
Perc diff between residential median 2019 certified and 2019 board bills:
(5068.47-4565.77)/4565.77 = .11

Other figures include the percent differences between median commercial bills at different stages:
Perc diff between commercial median 2019 mailed and 2018 board:
(70219.01-55332.65)/55332.65 = .27

Perc diff between commercial median 2019 mailed and 2019 board:
(57906.71-70219.01)/70219.01 = -.17