Add option to generate locations only based on activity staypoints

[henrymartin1]

I think that the way we define staypoints it would make a lot of sense to create locations only based on staypoints that are activities. For this we could include an activity_only option in the generate_location function.

E.g., in the paper we write:

Activity staypoints represent individual visits at places that are significant to the visitor. Trackintel models
these significant places using the location class to enable the characterization of the place that is visited.

[henrymartin1]

@hong2223 and I discussed today that having this feature is a good idea, however it should be optional as it requires staypoints to have an activity flag.