Allow to pass a specific reference vehicle to openGap()

[leoluecken]

Currently, open gap always considers the current leader on the own lane as a reference.
Extend this to have arbitrary reference vehicles, which may be on neighboring lanes and which may not be direct leaders.

[leoluecken]

@dimikout3, @vmintsis: If you have any specific requirements on that, please add them here

[leoluecken]

Uh, just noticed that an ACC-Model modification slipped through with f699dcb, which should address #4861

[leoluecken]

Documentation still open (postponed since Wiki is down)

[leoluecken]

docu done