Removed implicit surface functionality.

This hasn't been used in production for a long time, and is considered superceded by functionality elsewhere, in particular OpenVDB.  I've kept the files in the repository for now by just moving them to the contrib directory, so they can be found easily by interested parties if they'd like to use/copy/study/fork it.
Merge pull request #289 from johnhaddon/implicitSurfaceRemoval

Removed implicit surface functionality.