Export CSV for regional (multi-origin batch) analysis

[abyrd]

I've performed this kind of output manually a few times by merging shapefile tables. Doing it manually is not too complex, but involves a lot of back and forth between QGIS and a spreadsheet cutting and pasting, which then requires a lot of double-checking, which becomes time consuming. It should be straightforward to produce this output automatically.

We'd need one line per origin point, showing the min, avg, and max accessibility to each destination category in the destination point set, for the chosen time cutoff. This is the same as the current SHP output from regional analysis, but over the cartesian product of {min, avg, max} x {all destination categories}. And why not, across a range of cutoff times in 5-minute increments.

[abyrd]

Of course this doesn't need to be implemented in this particular repo, it will probably be part of TAUI or another repo. But I'm including it here among the other user feature requests, to be checked off as we implement in our new version of the UI.