ENH: multiple surfaces, self-intersection in surfaceFeatureExtract (issue #450)

- If the dictionary is named 'surfaces', a 'surfaces' entry is mandatory.
  This is a list of wordRe, which is used to load multiple surfaces from
  constant/triSurface directory.

- Other dictionaries may contain a 'surfaces' entry.
  In which case the behaviour is as above (loading multiple surfaces).
  The dictionary name will *NOT* be taken as a surface name itself.

- Regardless of how the surfaces are loaded or features extracted,
  an additional selfIntersection test may be used.

  Eg,

    surfaces
    {
        extractionMethod    extractFromSurface;

        surfaces            (surface1.stl surface2.nas);

        // Generate features from self-intersect
        selfIntersection    true;

        // Base output name (optiona)
        output              surfaces;

        // Tolerance for self-intersect
        planarTolerance     1e-3;

        extractFromSurfaceCoeffs
        {
            includedAngle   120;

            // Do not mark region edges
            geometricTestOnly       yes;
        }
    }