Use enum values for `atSource`/`atTarget` and `sourceToTarget`/`targetToSource` values

[louisgreiner]

This PR (#828) introduces something that should make the code clearer: TrainrunSectionNodeAnchor.Source and TrainrunSectionNodeAnchor.Target. This intends to represent either the "source node" or the "target node" location.

We could use that widly in the code, to remove the atSource or similar things.

Also, we could add something similar to remove the "sourceToTarget" and "targetToSource" strings. I propose TrainrunSectionDirection.sourceToTarget and TrainrunSectionDirection.targetToSource

Implementation plan:

• use TrainrunSectionNodeAnchor everywhere
• use TrainrunSectionDirection everywhere