Single location selection

[willowo]

A user may want to be able to see all trade routes that go through a single country, regardless of whether that country is an exporter, importer or transit point. This helps them get a complete picture of the trade going through one country (e.g. what role does China play in the ivory trade?")

Currently you can only filter countries (locations) by their role. It would be great if there was the option replace the multiple location dropdowns with a single location dropdown.