Small adjustments to swapmove generator

Use random.sample for sampling 2 symbols directly. Also adjusts some initialization logic in the swap move tracker.

Also, it seems like this ExponentialFilter is unused in the MC. Perhaps it's something which could later be introduced as an observer, if it's still interesting, but it doesn't seem like something that belongs in the MC by default. Admittedly, I have no idea what it's doing, but if taking it out breaks 0 tests, and searching for it yields no other results in our code, I'll go ahead and take it out of the MC. I left in the class itself for now.