Creating a new entity called a trip. This will group together any travel
accommodation and conferences that happen together on one trip.
A trip is assumed to start when leaving home and finish when returning
home.
The start date of a trip in is the trip ID. The date is written in ISO
format.
This assumes there cannot be multiple trips one one day. This assumption
might be wrong, for example a morning day trip by rail, then another
trip starts in the afternoon. I can change my choice of using dates as
trip IDs if that happens.
Sometimes during the planning of a trip the start date is unknown. For
now we make up a start date, we can always change it later. If we use
the start date in URLs then the URLs will change. Might need to keep a
file of redirects, or could think of a different style of identifier.
Trip ID have been added to accommodation, conferences, trains and
flights.
Later there will be a trips.yaml with notes about each trip.
