I was going to write the DerivativeAccumulator code today, since (as
Daniel also points out) it's useful regardless of whether we want
weights (e.g. if we want to sum derivatives as plnp rather than just p).
But if somebody else wants to write it, let me know!
OK, so there is a very simple first approximation to a
DerivativeAccumulator class in r553, and RestraintSets as of r554 will
weight their child restraints (and have a set_weight() method). We can
break/fix this with future commits. ;)