Custom symmetry restraints not used

[amjjbonvin]

Module/Workflow/Library affected

rigidbody, flexref, emref, mdrej

Expected behavior

When symmetry restraints are turned on and a custom symmetry file is defined (symtbl_fname) those restraints should be read and used.

Actual behavior

While symtbl_fname is correctly defined, it is not read anywhere

Suggestions on how to fix it

Modify symmultimer.cns to read the restraints if symtbl_fname is defined

Additional context

Currently intending to use those restraints for a CG application.
Will fix the issue in the CG-HADDOCK branch first, which should soon be merged into the main branch.