Add isotropic atom to U(IJ) restraint

[pascal22p]

Some corner cases would make this very useful

[richardicooper]

It would be more faithful to the model to convert EADP to constraints. RIDE can be used to do the job. For isotropic it will work provided both atoms are isotropic (EADP has same restriction). Our main problem with the guided refinement mode in CRYSTALS is that it currently can't co-exist with additional user defined constraints (it can set up occupancy constraints and standard rides for H atoms, but not these custom EQUIVALENCE directives). Needs some designing..