Add population metadata

[nvkelso]

We have a line item for Population (wof:population), but we also need to set:

• [ ] Population_rank is a calculation based on population (wof:population_rank as int). This should auto update when the wof:population is updated... but can be set manually when wof:population is not provided.
• [ ] Will also need to update src:population (string) when moderated by "wof" or to indicate it's sourced from another provider.

[nvkelso]

@stepps00 can you add detail here, please?

[stepps00]

When updating population data (wof:population), we should also set the following properties:

• Population Rank
• Population Source
• Population Source Date

Here's some Python logic to determine population_rank based on the population data: https://github.com/whosonfirst/whosonfirst-cookbook/blob/master/scripts/108_gn_import.py#L715-L748