COMP: Add VTK6 compatibility, without killing VTK5 compatibility

This patch does the work to permit building BRAINSTools with either VTK 5 or VTK 6. 

There appears to be a problem introduced -- one test in BRAINSSurfaceTools is failing.  I suspect the problem comes in VTK6 because the code is only changed in a very innocuous way.

This could be merged immediately. It would be good though to get a second pair of eyes on how I implemented the changes/backwards compatibility.

I will keep looking at the VTK6 regression.