TODO: Change TSPLibTour class

[jkinable]

Currently, the TSPLibTour class (tspLibReader package) which models a tsp lib tour is not very practical in use:
-Tours need to be added and accessed via the TSPLib instance
-Tours don't keep track of their length. To get the length of a tour, one needs to invoke tspLibTour.distance(tspLibInstance); which recomputes the length of the tour with respect to the given TSPLib instance.

Proposed changes:
-Separate TSPLibInstance and TSPLibTour classes, thereby making them independent:
--Move the code which reads/parses a TSPLibTour from the TSPLibInstance to the TSPLibTour class
--Add a field to TSPLibTour which stores the length of the tour
--Remove the addTour() method from TSPLibInstance; this doesn't really make much sense
Note: These changes have implications for the other classes in the tspLibReader package. This has to be checked.