Fix dynamic pclasses, and make them the default for virtual nodes
again. One of the fixes changes the way in which we iterate through pclasses in find_pnode(). We used to treat the vector like a ring buffer, and start (randomly) someplace in the middle. This turns out to give some bad statistical properties when doing dynamic pclasses, since long chains of disabled pclasses will cause some pclasses to be selected more often. My old hack of just hopping around randomly in the disabled-pclass case was bad, because it's hard to tell when you've actually tried all the pclasses - so, we were getting false negatives where it was looking like there was no place available where we could map a vnode, which turned out to have worse effects than I had thought. So, now, we make a list of all the indices and randomize the order, then just iterate through that list. We also now count the number of pclasses that are enabled at every temperature step, and adjust the neighborhood size to remove them. This makes dynamic pclasses quite a bit faster - it cuts the time by 30% - 50% for my test case. Cleaned up find_pnode() by removing some #ifdef's that we don't use, and probably will never want to again - this makes the function almost readable!