Use geodatasets for examples instead of geopandas

There has been a change in the example data that is/isn't shipped with `geopandas`, in particular the various "naturalearth" datasets. We should replace these with use of other examples from `geodatasets`. The only place in datashader that this relevant is in `8_Polygon.ipynb`. See holoviz/geoviews#635.

A suitable replacement is something like
```python
import geodatasets as gds
import geopandas
us_counties = geopandas.read_file(gds.get_path('geoda health'), include_fields=["statename", "countyname", "cty_pop200"])
```
which works locally but gives some obscure projection error in CI. I don't wish to hold off the 0.15.0 release any longer, so this can be dealt with after that.

Start with 8_Polygon.ipynb and the proposed geodatasets example using geoda health. Reproduce the example locally, then investigate the projection error reported in CI. Done means the notebook no longer relies on geopandas naturalearth datasets and passes CI.

Written by the indexing model from the issue text.