Fix time estimations in train master

[ferrous26]

Time estimations are horrendously (but consistently) off in train master for some reason. Position updating seems to be correct, so it must be something else.

This is a good opportunity to refactor some train master logic. Train master needs mission control to compute a couple of things:

• the distance between two track locations

Given two (ordered) track locations, A and B, find the distance from A to get to B.

• the distance between a track location and a negative offset

This is similar to get_position_from but moves backwards instead of forwards.

[ferrous26]

This may need to be done concurrently with moving short moves. An overall refactoring of train master.