LCMAP example: constructs a time coordinate from info found in the filenames

[rsignell-usgs]

I didn't remember this quite right -- we didn't read the original geotiff files here because kerchunk doesn't create xarray spatial
coordinates yet when reading geotiff. So we first convered the geotiff files to netcdf, and it's the NetCDF files we are using in this example:

https://nbviewer.org/gist/cea8e1ee16e318e05c128353482836b9

It would be good to add this example to kerchunk (it also shows accessing an S3 endpoint that is not on AWS!)