Support multiple primary parameters (taxonKey, datasetKey, country, publishingOrg, publishingCountry) for density tiles

[MattBlissett]

A tile for the western hemisphere showing:

House sparrows: http://api.gbif.org/v2/map/occurrence/density/0/0/[email protected]?srs=EPSG:4326&bin=hex&taxonKey=5231190&style=purpleYellow-noborder.poly

Mackrel: http://api.gbif.org/v2/map/occurrence/density/0/0/[email protected]?srs=EPSG:4326&bin=hex&taxonKey=8596&style=purpleYellow-noborder.poly

Both: http://api.gbif.org/v2/map/occurrence/density/0/0/[email protected]?srs=EPSG:4326&bin=hex&taxonKey=5231190&taxonKey=8596&style=purpleYellow-noborder.poly

See mailing list discussion: https://lists.gbif.org/pipermail/api-users/2018-August/000555.html

[timrobertson100]

This method and possibly this method should allow one to merge tiles. I'm not sure of all the implications, but I'd think we can someone simply get each tile, and then collect them into a single one so this might not be too invasive a change. I had this working at some point, but it might have been in the POC and then overlooked in the rewrite for production.

I think collection would need to complete before binning.