Materialize has _weird_ defaults

Closes #79

This check is already present in the backend; now the frontend knows about it
too.

travelynx now always calls checkout with the database-provided arr_eva,
so the edge case in which it provided a station that is not actually on the
route of a train can no longer appear.

HAFAS stations do not necessarily have ds100 codes

was removed from $self->checkin in 1074f248