ENH: 3DErrorMetric was renamed and made to compiled as a Slicer4 extension

-Renaming 3DErrorMetric into ModelToModelDistance
-ModelToModelDistance compiles both with VTK5 and VTK6
-Addition of tests for ModelToModelDistance
-ModelToModelDistance only saves the distance between the points, not between the cells
-Changed <category> in XMLs to match SPHARM's category: Shape Analysis