add nhgis interface

[knaaptime]

nhgis has a new api for extracting data from their system. As far as i can tell, it's currently asynchronous like their web extracts because it takes awhile for their backend to process the request

IPUMS Metadata and Data Extract APIs, allow you to submit a JSON request to describe the data you want, monitor our progress building your data extract, and download your data when it is ready.

this likely wont be ready for some time, but as the cenpy wrapper gets up to speed, it will be easy to include this alongside the pre-harmonized LTDB and NCDB sources.

One thing to mention is that our experience with NHGIS boundary files in the past has revealed tons of topological problems that result in invalid weights matrices.

[knaaptime]

https://developer.ipums.org/docs/workflows/create_extracts/nhgis_data/

[knaaptime]

the async API they provide doesnt fit well into the workflow, so closing this. If folks want to use NHGIS data with geosnap, its better to collect the data manually ahead of time