The N-state model code has been debugged a lot since it was copied and modified for the frame order
analysis, but those fixes have not been migrated to the frame order analysis. The fixes to
_minimise_setup_pcs() and _minimise_setup_rdcs() have now been migrated over. To help the data
checking, the _opt_uses_align_data(), _opt_uses_pcs() and _opt_uses_rdcs() methods have also been
ported over. Finally, a new check has been added to raise a RelaxError if no RDC or PCS data is
found.
In addition, the _minimise_setup_atomic_pos() has been created to decouple this from the PCS data
assembly (this also now matches the N-state model).
