fix DST change issues

[miklcct]

The GTFS specifications specifies that the time is defined as starting from noon minus 12 hours, in order to avoid a DST change happening on a service day. Therefore, trains departing before the DST change, i.e. 02:00, should be considered part of the previous service day in GTFS:

• A train scheduled to depart 01:05 on March changeover day will run on GMT for its whole journey, while a train scheduled to depart 02:05 will run on BST.
• A train scheduled to depart 01:05 on October changeover day will run on BST for its whole journey, while a train scheduled to depart 02:05 will run on GMT.

This doesn't have any special handling on skipped / additional trains in the skipped / additional hour on the Night Overground services, which the handling is yet to be confirmed.

Fixes #86 .