message: only extract CrossLinkRestraints when needed
diff: /**
  - * Copyright (c) 2018 mol* contributors, licensed under MIT, See LICENSE file for more info.
  + * Copyright (c) 2018-2019 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
  @@ -89,6 +89,7 @@ function createCrossLinkRestraint(unitA: Unit, indexA: StructureElement.UnitInde
  function extractCrossLinkRestraints(structure: Structure): PairRestraints<CrossLinkRestraint> {
  const pairs: CrossLinkRestraint[] = []
  + if (!structure.models.some(m => IHMCrossLinkRestraint.fromModel(m))) return new PairRestraints(pairs)
  const n = structure.units.length
  for (let i = 0; i < n; ++i) {
  @@ -96,9 +97,11 @@ function extractCrossLinkRestraints(structure: Structure): PairRestraints<CrossL
  extractIntra(pairs, unitA)
  for (let j = i + 1; j < n; ++j) {
  const unitB = structure.units[j]
  + if (unitA.model === unitB.model) {
  extractInter(pairs, unitA, unitB)
  }
  }
  + }
  return new PairRestraints(pairs)
  }
  