Decouple local search operators

[N-Wouda]

Similar to #32 for the crossover operators: we should decouple the operators from the LocalSearch (LS) class to allow testing their added value. This is difficult, because the LS class is a bit of a beast. I attempted to clean this up before in #24, but that eventually did not work out. Another attempt might be more successful, however.

First, we need to determine a good signature for the local search operators. What must they minimally know about the current neighbourhood? What do they use currently? Can we pass that data in as arguments, and somehow get a (possibly improved) solution out?

[N-Wouda]

@LiekevdHeide @LuukPentinga if you're looking for a useful thing to do (other than reviewing PR's!): this is quite important and decidedly non-trivial :-).