Fix bug in geojson example, and rename some variables in examples for clarity

[jake-low]

This PR does two things:

1. Changes EmptyTagFilter().apply_to() to EmptyTagFilter().enable_for() in the GeoJSON example, so that the example works (see first commit).
2. Renames some variables in the examples (see second and third commits). I found it confusing that the name o was used both for the osmium library and also for an OSM Node/Way/Relation, such that the latter sometimes shadowed the former. So I changed the examples to refer to the library by its full name, and to refer to OSM objects as n/w/a/r if their type is known (e.g. when they are arguments to the node/way/area/relation handler functions). I tested all of the examples to make sure they still worked after the changes.

Let me know if you have any feedback on these changes, and thanks for maintaining pyosmium.

[lonvia]

Looks all good to me. I had already stepped away from the o alias for osmium in the cookbooks, precisely for the reason you mention. So definitely a step in the right direction. Thanks.