Calculate bbox from the filter region

[rddesmond]

In cases where the --region-include is given and given before --region-exclude, use that to calculate the bbox so that it can be an optional parameter. This would allow a search based on a AOI polygon, and be much easier to specify.

This could be from the first region-include, but it would be better to calculate the complete filter region and use that.

[rddesmond]

This could be related to approaches to #214. In that, one approach could be to wait to calculate the grid at the last minute (which means storing the global polygons). (Another approach, calculating a common grid, isn't applicable here. Of course others probably exist too.)