Add tree levels to the vtk output

This commit adds the option to output the octant levels to the vtk visualization file. 

I have tried my best to stick to the coding style put forth in the pre-existing code. I have tested this in ASCII and binary mode as well as in serial and parallel mode and everything seems to work as intended. However, I did not run any of the test codes nor did I run the modified source through the indent script (as I was not entirely sure how to use it.)

I understand if this functionality is not something that you want as a part of your project but I thought it might be valuable and I figured I'd make the pull request available for you to consider.

If you are interested in this but would like me to modify anything first please let me know and I'll be glad to do what I can.

I'm excited about the possibility of using this software in some of my work!

Thanks!!