[WIP] Focal Statistics on Time Series Lesson

[rbavery]

This is the start of an episode with the following objectives:

• show how to query a time series by taking the total bounds of a GeoSeries (multiple polygons)
• read a spatial subset of a small time series using rasterio's windowed reading and Sentinel-2 COGs (working out problem here)
• computing focal statistics of the time series array using the farm field vectors
• plot summaries of the focal statistics, grouping results by field class

[rbavery]

closing in favor of https://github.com/carpentries-incubator/geospatial-python/pull/102