#4 Refactor particle length adjustment

[Juergen Bock]

The particle length adjustment procedure is rather obscure and probably buggy. In order to make it easier to test, the single method should be refactored into several that provide the different core functionalities required for length adjustment, i.e. computation of change probability, computation of actual length change, and performing the adding and removal of correspondences. The different methods should be designed for easy testing, and as decoupled as possible from the rest of MapPSO.