attempt to make SimilarityABC __getitem__ faster

gensim interfaces.py; work with tuple directly. just as slow

     def __getitem__(self, doc):
        import time
        # get similarities of doc to all documents in the corpus
        b = time.time()
        if self.normalize:
            doc = matutils.unitVec(doc)
        allSims = self.getSimilarities(doc)
        a = time.time()

        # return either all similarities as a list, or only self.numBest most similar, depending on settings from the constructor
        if self.numBest is None:
            return allSims
        else:
            c = time.time()
            top = set()
            lowest = (None, 1)
            # pairs of (docNo, score) tuples
            for el in enumerate(allSims):
                if el[1] <= 0:
                    continue
                # do the initial fill
                if len(top) != self.numBest:
                    top.add(el)
                    if el[1] < lowest[1]:
                        lowest = el
                    continue
                # replace an old value with a better one in the full list
                if el[1] > lowest[1]:
                    top.add(el)
                    top.remove(lowest)
                    lowest = (None, 1)
                    for t in top:
                        if t[1] < lowest[1]:
                            lowest = t
            print top
            ret = sorted(top, key = lambda item: -item[1]) # sort by -sim => highest cossim first
            d = time.time()
            print "SIM: %f     SORT: %f" % (a-b, d-c)
            return ret