ENH: ensure face, triFace and labelledTri all work consistently (issue #294)

- triFace() now initialized with '-1', which makes it behave
  equivalently to face(label).

- supply default region=0 for some labelledTri constructors.
  This allows labelledTri to work more like a triFace and makes it
  easier to use in templated methods and eases conversion from
  triFace to a labelledTri.

- labelledTri(const labelUList&) can now be used when converting
  from a face. It can have 3 values (use default region)
  or 4 values (with region).

- face, triFace, labelledTri now all support construction with
  initializer lists. This can be useful for certain types of code.
  Eg,
      triFace     f1{a, b, c};
      face        f2{a, b, c};
      labelledTri f3{a, b, c};

  Work without ambiguity.
  Also useful for templated methods:

      FaceType f{remap[a], remap[b], remap[c]};