Resolve "Add comparison ops for events"

Some cleanup of comparison module and add comparison operation for TermTime events to Booker.

Booker has no Id field for bookings to match against, so using a formatted Description value to link with TermTime activity id and instance.

Tests for function, including testing invalid data which could be used as template for similar tests for other comparison functions when doing #21.

May be easier to review one commit at a time

Closes #18 (closed)