[IMP-dev] CoordinateRestraint

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: SphericalDistanceRestraint(Model*, ParticleIndex (or Particle*), Float x, Float y, Float z, ScoreFunc *f);

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).