Fix uninitialized variable bug in libHilbert.

This was causing undefined behavior on Linux in my simple libHilbert
inverse mapping tests, but seemed to work fine on OSX.  Not sure if it
would affect other libmesh code...
Adding Hilbert test/debugging code.

This should not be merged into master...

Converting back and forth between CBigBitVecs (instead of involving
CFixBitVecs) seems to work much better.