Use versions to indicate changes to data year w/ documented update schedule

[elipousson]

If this package is ever going to be useful to other folks than myself, I need better documentation for changes to the underlying data. I just updated the MTA Bus stop and bus route data but, in the process realized that I missed the 2021 data release (no longer available online although I can likely request it) and I also have no place to indicate the current year for any of the datasets beyond the title.

There is a similar issue with my bcpss package so I need to investigate the best approach to implement in both cases.

[elipousson]

The versioning approach outlined in the Natural Earth Vector repository seems like it may be a helpful model to use the overall package versioning to track changes to the underlying data: https://github.com/nvkelso/natural-earth-vector#versioning

[elipousson]

If mapbaltimore is also using data from related packages like bcpss, I could do detailed tracking of changes on the source packages and link to those from mapbaltimore. It wouldn't work in all cases so I'd need to document how I handled version tracking for different datasets.