ECCE-QM: dummy atoms and geometry constraints (for constrained optimisations in teaching labs) #309

So that constrained-geometry labs (e.g. mapping the ammonia inversion potential with a dummy atom on the C3 axis, #298) can run with the built-in program, without NWChem:

1. Dummy atoms: read X in the geometry block with no nucleus, electrons or basis functions; leave it out of the SCF, the DFT grid, Mulliken etc.; keep it in the geometry trace so atom numbering matches the Builder.
2. Geometry constraints in the optimiser: fixed bond lengths, angles and torsions, and fixed atoms (including dummy atoms), enforced at every step (e.g. projection of the constraint gradients from the BFGS step, or Lagrange multipliers); new input keywords documented in the help.
3. ECCE writes them: ai.ecceqm writes the Builder's constraints (as for NWChem and ORCA) instead of refusing dummy atoms.
4. Validation: constrained NH3 + X at fixed X–N–H angles (HF and B3LYP/6-31G*), energies and geometries along the inversion path against NWChem and ORCA.
   Planned for 9.0.1, together with Geometry constraints and dummy atoms: make them discoverable #306 (constraint workflow, including picking angle/torsion atoms in the right order).