Problem and heuristics refactor

[jcoupey]

For future evolutions, the code structure should allow to:

• define and expose different types of problems in an extensible way;
• add dedicated solving strategies as "plugins", or somehow bind each strategy to the relevant problem.

The current TSP-solving code should be refactored as the first plugin. This probably means adding classes to describe objects that don't exist as such so far (e.g. solutions, maybe routes, jobs, vehicles...).