ENH: add more direct methods for getting meshEdge (PrimitivePatch)

- refine definition of patch boundary faces to distinguish between
  boundaryFaces() and uniqBoundaryFaces().

  * boundaryFaces() for edge to face lookup on boundary edges.
  * uniqBoundaryFaces() for accessing quantities such as face areas
    or eroding an outer layer

ENH: LabelledItem container, replaces unused 'Keyed' container

- method names in alignment with objectHit, pointIndexHit etc.
  Top-level name aligns with labelledTri.