`gol build`: Discard (or fix) invalid multipolygon relations

[clarisma]

Invalid multipolygon relations are currently responsible for several issues (clarisma/geodesk-py#29, clarisma/geodesk-py#30, clarisma/geodesk#52) -- gol build should remove (or fix) them.

Proposed approach:

• Keep broken MP relations, but don't treat them as areas; store them as regular relations instead

• To be considered an area, an MP relation must have at least one member way with role outer

• An MP relation will be included as an area even if its geometry is invalid

See also #103, #107