Clean up vehicle definition implementation

[Janekdererste]

This is related to #87 and #76 .

I am not happy with the current vehicle definition. This should work differently somehow.

We always want agents to travel by vehicle. If no vehicle types are defined we can use default ones. During the simulation this should not be an Option but should rather be present as a preprocessing stepj.

[Janekdererste]

This will be resolved by #99