extract AdaptiveCellAdapter from QuadrangleAdaptiveCell
Add generic parameter to AdaptiveCellAdapter

  - also modify QuadrangleAdaptiveCell accordingly
add AdaptiveCellAdapter.linkEdges()

  - let QuadrangleAdaptiveCell and QuadrangleAdaptiveCellFactory use it
  - also improve QuadrangleAdaptive.createChildrenFromBisectionedEdges() slightly