enabling custom torsions in DihedralConstraintGenerator (#6458) * changes allow for use of custom dihedral angle with declaration of custom torsion atoms. * current functionality is maintained * Adding an integration test for the DihedralConstraintGenerator. The newly revealed functionality for specifying a custom dihedral_angle is shown to work. Previous functionality that creates constraints based on the current conformation is also shown to still function as intended.

• Summary
• code_quality.clang_analysis 「1」
• code_quality.clang_tidy 「5」
• integration.addsan 「2」
• integration.mpi 「1」
• integration.release_debug 「5」
• integration.tutorials 「1」
• integration.ubsan 「4」
• integration.valgrind 「5」
• integration 「3」
• maintenance.documentation
• scientific.ligand_scoring_ranking
• scientific.mp_f19_decoy_discrimination.debug
• scientific.mp_lipid_acc
• scientific.protein_data_bank_diagnostic.full