Enumerations for _Constraint_file.Constraint_type (An open enumeration)
`chemical shift` | enumeration_text
`chemical shift anisotropy` | enumeration_text
`coupling constant` | enumeration_text
`dipolar coupling` | enumeration_text
`distance` | enumeration_text
`hydrogen exchange` | enumeration_text
`intervector projection angle` | enumeration_text
`line broadening` | enumeration_text
`nucleic acid base planarity` | enumeration_text
`nucleic acid dihedral angle` | enumeration_text
`nucleic acid other kinds of constraints` | enumeration_text
`other angle` | enumeration_text
`protein dihedral angle` | enumeration_text
`protein other kinds of constraints` | enumeration_text
`protein peptide planarity` | enumeration_text
`pseudocontact shift` | enumeration_text