Generalize on_best to on_outcome about alns HOT 4 CLOSED

I think that makes sense. To not break the current code, I think we should consider an addition:

def on_best(self, func):
    self.on_outcome(func, Outcome.BEST)

def on_outcome(self, func, outcome):
    self._on_outcome[outcome] = func

from alns.

Since the actual amount of code needed for this is very small, we could also just do on_best, on_better, on_accept, on_reject as explicit methods, rather than expose the Outcome enum to the users.

from alns.

This comment is relevant here:

Should we create an IntEnum for the evaluation outcome, and pass that to update, rather than a meaningless integer s_idx?

I'm thinking of

import enum

class Outcome(enum.IntEnum):
    BEST = 0
    BETTER = 1
    ACCEPTED = 2
    REJECTED = 3

Originally posted by @N-Wouda in #74 (comment)

from alns.

This one's pretty easy as well. I will try to finish this one tonight, so we can submit our JOSS paper (#58). @leonlan could you review the resulting PR?

from alns.