Remove std::shared_ptr from class Stop?

[matdahl]

Is there a reason why the member request of the class template Stop is a std::shared_ptr?
As the usage would be much easier if it would be just a normal pointer, I would propose to replace this by an ordinary pointer Request<Loc> * otherwise.

Furthermore, the default constructor Stop<Loc>() currently lead to undefined behavior since estimated_arrival_time and occupancy_after_servicing are not initialized in this case and therefore contain random values when constructing a Stop this way. Is there a use case for the standard constructor of Stop?
Otherwise, it would be safer to remove it and force people to use the other constructor that requires all needed parameters.