It seems entirely useless to me as written as it can only restrict
coordinates relative to the origin. I propose getting rid of it and
instead providing, as needed, SphericalDistanceRestraint (restraint
the distance from a fixed point), CoordinateRestraint (which restricts
a coordinate relative to a fixed value) etc. The names could use some
work.
My suggestion for SphericalDistanceRestraint (which is what I want) is:
Then you can enforce any spherically symmetric restraint on a set of
particles (in my case enforcing that they are all in some sphere
bounding the system).