Anyway, I am fine either way. If we want to switch it I can do it
as part of moving the base classes to the kernel.
I think that it makes sense for a function like modifier::apply and
restraint::evaluate to be const - but this is major change in the
interfaces of base classes - so we should get agreement from Ben as
well for that.
The bigger question is whether there is any existing code that takes
advantage of them being non-const. I suspect not, and if not, it is
actually a very minor change (due to the IMP_RESTRAINT macro and the
few SingletonModifiers).