Add support for exposure export by named region

[Paul Henshaw]

Instead of selecting a bounding box, it would be useful to be able to obtain the grid points for a named region; e.g. all cells in the municipality of Zug.

For the interactive web-UI I suspect we will also need a means to search for regions by name and allow the user to choose when there are multiple matches.

For the web API we will probably need to accept either region IDs or require the name/iso code of all parent regions in order disambiguate.

[Paul Henshaw (paul-sl-henshaw)]

See also https://bugs.launchpad.net/oq-platform/+bug/1221816 "Extend exposure export tool to provide exposure for a list of points provided by the user"