remove interface Comparable from IntegerLeastSquareSolution

The IntegerLeastSquareSolution implements Comparable but this is not strictly needed from an API point of view. The sorting order of various solutions is more related to the algorithm that solves the integer least squares problem and is generally related only to some properties (the squared distance to the float solution), not to the other properties (the integers).

Having IntegerLeastSquareSolution implement Comparable also implies implementing equals which itself involves implementing hashCode. However, from a logical point of view, these two methods should depend on all properties, not only on the squared distance, and therefore be inconsistent with compareTo.

It would be more consistent if IntegerLeastSquareSolution does not implement Comparable but the solver uses a separate and dedicated Comparator for sorting.

This change is however backward incompatible, so it can be done only for a major release.