implement reference model checks

[tkrojer]

• no LIG/DRG ligands allowed
• reference CIF file fits non-standard ligand*
• highlight status in model selection dropdown, e.g.
  model 1 - OK
  model 2 - PROBLEM

*) it has happened in the past the users first generated ligand PDB/ CIF file with one program (e.g. ACEDRG), then placed the ligand and refined. But later, for various reasons, generated a CIF file with another program, but did not update the coordinates of the ligand in the protein-ligand PDB file. Refinement failed in these cases because the atom names did not match.