Allow default construction of osmium::io::File
Allow the type of an item to be set by its children.
Add size() method to Box and box contains() function
Add Area datatype

Adds a new Area datatype which needs some more changes elsewhere. WayNode
was renamed to NodeRef, because they are not only for Ways any more.
There are new OuterRing and InnerRing objects, which needed some changes
to the way Collections work.
Add support for creating Multipolygon geometries from Areas.

OGR and WKT geometries can be created from Areas. There are some tests to check
the WKT multipolygons.
Add first stab at multipolygon code.

This commit adds all the scaffolding needed before we can do the actual
multipolygon assembling. So it does the collection of all the members of
multipolygon relations, deciding when we have all the data needed and then
calling the actual assembly. The assembly code currently is only a placeholder
that does as very simplistic (and often wrong) multipolygon assembly. There are
two test programns that dump out the created areas (osmium_mpdump) and create
OGR geometries from the assembled areas (osmium_toogr2).
Add multipolygon test data