Add a command to set .country on areas (and assess ambiguities)

[mhl]

This command:

• Iterates over every area

• Takes an arbitrary point within every polygon in that area

• Finds all the O02 (country-level) areas that contain those points

• If there's exactly one of those, sets it as the country on the area

• Otherwise sets it to a special '?' country, and outputs an error message describing the ambiguity

The plan is to run this command (without --commit) to assess the number of areas that have ambiguous enclosing areas, and then decide what do about those cases. (e.g. we could change MapIt's data model to accommodate multiple countries for an area, making sure not to break the existing API; we could pick the smaller of the ambiguous regions (typically more useful information); we could use the special '?' country for real.)

[coveralls]

Coverage decreased (-2.3%) to 64.863% when pulling 5dd4af31247e111497dd72e0e1c3a9e667851fbf on assess-ambiguous-countries into 494c0e8b242bc73d670c84efc911bce631a39b89 on master.

[coveralls]

Coverage decreased (-2.2%) to 64.937% when pulling cf9df1403ec4a94e6c275f22f433b9a3cccb2287 on assess-ambiguous-countries into 494c0e8b242bc73d670c84efc911bce631a39b89 on master.