See https://www.rfc-editor.org/errata/eid4164
The original RFC's appendix was missing a part of the calendar data used
in the examples. This will become relevant when adding tests for
retrieving recurring events.
This is not yet complete (see TODOs in code), but basic filtering of a
list of CaledarObjects works.
Includes test data from the RFC, which allows to use the RFCs examples
as test cases.