Implement ability to autocrop dataset to data extent

[brendan-ward]

Some inputs have large areas of nodata. Such a waste. Figure out the bounds of the actual data, and allow cropping to that.

[brendan-ward]

Consider implementing upstream in rasterio as well.

[brendan-ward]

Added in #9daefdabfba4e1fe53216d5d5c14b569dba4ace0

Depends on rasterio/rasterio#506

And will require a new version of rasterio.