Drawing a box to select places to filter by

[rybesh]

A longstanding item on the wishlist has been to enable drawing a box on the map and showing periods with spatial footprints within the box. We've put this off because finding the intersection of a box with the spatial coverage of ~7000 periods in pure JS is likely to be very slow without some kind of dedicated spatial index.

However yesterday it occurred to me that another option could be to simply add box selection to the existing UI for selecting gazetteer places to filter by. There are a lot fewer records in the gazetteer, so this could be pretty quick. It would basically work just like the existing place selector, except that rather than searching for places by name, you would draw a box on the map to get a set of places to filter by.