Introduce filter handlers

[lonvia]

Filter handler can be used to drop irrelevant OSM objects before they are handed into Python callbacks, thus significantly speeding up processing. They can be either used with apply (order matters!):

osmium.apply("osm_file.pbf", osmium.filter.KeyFilter('amenity'), MyAmenityHandler())

or handed into the apply_file/buffer functions of the SimpleHandler:

MyAmenityHandler().apply_file('osm_file.pbf', filters=[osmium.filter.KeyFilter('amenity')])

The latter has the advantage that it will correctly work together with the location cache and area handling.

Filters are always implemented in C++. Allowing filters to be defined in Python would defy the purpose of avoiding callbacks into Python. There are two filters defined right now:

• EmptyTagFilter filters out all objects without any tags.
• KeyFilter only lets objects pass that have one of the given keys

Processing a Switzerland extract with the EmptyTagFilter reduces processing time from 40s to 6s.