Allow user to label every Nth contour line [1375]

Guys - this is all documented on RedMine:

https://mcidasv.ssec.wisc.edu/issues/50

And in McV Inquiry #1375:

http://dcdbs.ssec.wisc.edu/inquiry-v/index.php?inquiry=1375

NOTE: you will need to bring in VisAD changes from VisAD Pull Request #22 before you can test these changes!