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!