Support for ply "triangle" element for tri meshes

* Fix ply type detection, which was broken in the previous commit due to
  an errant strcmp() test.

* Add support for reading triangle meshes with face elements given by

  element triangle
  property v1
  property v2
  property v3

* Support for reading colors from the vertex element as well as a
  separate color element.

* Extra logging of ply read anomolies

* Add test case/example for ply file with triangle mesh