Census example spatial indexing

[ablythed]

In the version of the census notebook in #166, I removed the deprecated datashader.spatial import and therefore had to find an alternative to spatial.read_parquet() for the second cell. I tried to replace it with spatialpandas.io.read_parquet_dask() but that gave the following error: Pinning fastparquet latest version didn't help, and I'm unsure of what else to try. In the final version in #166, spatial indexing is removed altogether and that step is replaced with dd.read_parquet().