BUG: minor regression. STL reading in double, not float (issue #491)

- By definition, binary STL uses float (not double) when reading.
  The ascii STL should be the same. This reduces memory overhead when
  loading files. The older triSurface reader had float, the surfMesh
  reader had double, but now has float.

- Inconsistency in the STL merge-tolerances between triSurface reader,
  surfMesh reader and WM_SP vs WM_DP. Now use consistent tolerances
  conrresponding to 10,100 * doubleSMALL.

- Similar float/double code adjustments for TRI format since this is
  very similar to the STL reader and had a similar inconsistency between
  the triSurface and surfMesh version. The AC3D reader still uses
  double when reading, but this can be revisited in the future (and can
  then remove the stichTriangles method too).