year of census data

Now that census geographies are being updated in the catalogue to [2021](https://catalogue.data.gov.bc.ca/dataset/1cb07568-0b43-421a-a658-e69f7c1a47a0) we will need to think about how to manage this. I see three options:

- Replace the 2016 census geographies with the 2021 geographies and simply commit to having the most recent version in the package
- Add an argument to `census_*` that specifies the year of the census with the current boundary at the default (create new functions `census_2016_*` and `census_2021_*`)
- Bind the two datasets together and manually filter for the census year of interest