Instead of using the default yaml loader make sure to use our customised
loader. Also move the loading stage into our test cases as this should
be part of the test case to ensure that it functions for the individual
test case.
- Add copyright headers to files that did not already have them
- Update the year to the last year the file was modified in files with
existing headers.
The current test was designed to test the removed `Agenda` class, we now
use use an `AgendaParser` so update the tests to test its functionality
instead.
