Remove virtual inheritance

AFAIK virtual base classes are only necessary in the context of multiple inheritance (see https://isocpp.org/wiki/faq/multiple-inheritance#virtual-inheritance-where).

Making IVehicle, IPedestrian, and IStaticObject inherit virtually from IEntity, which only make sense if we would like to communicate that the following Entity is possible - or actually what a user should do at the implementation side:

class ObstacleThingy : public IVehicle, public IStaticObject
{
   ...
};

I personally prefer composition over inheritance, so unless there are some objections, I'd prefer to remove the virtual relation.