Add datashader.SpatialDataFrame reader

[jsignell]

There is a special spatially optimized dataframe that we've been working on for datashader. It is a parquet file that is organized in such a way that datashader can ignore certain partitions when zoomed in. It would be nice to add support for reading to this class from parquet files if datashader is in the env.

Does that seem like a reasonable addition to this plugin? If so I could work on it.