Refactor multipolygon geometry conversion.
Add GEOS geometry factory.
Reorder classes in osm_object_builder.hpp.

In preparation for more changes.
Move create_tag_list functions into different namespace/include file.

They fit better in the osmium::builder namespace and are now named
build_tag_list, too.
Add build_way_node_list function and use it in tests.
Rename add_way_node to add_node_ref.
Refactoring WayNodeListBuilder, OuterRingBuilder, InnerRingBuilder.
Remove commas from end of initializer_lists.
Add functions to help build objects and test them.
Make most constructors "explicit".
Add some noexcept declarations.
Rename osmium::Object to osmium::OSMObject.

"Object" is a rather generic name for a class. And this fits with the existing
OSMEntity.
Add [[noreturn]] to not_found_error() function.
Use enum class wkb_type instead of bool in WKBFactory.
Make byte order marker enum class.
Rename remaining way_node vars to node_ref.
Make member functions in geometry factories const that can be const.
GeometryFactory code cleanup.
Add OGRFactory test.
Refactoring of OGRFactory.