BUG: avoid false positives on geometry check

Add an epsilon value to the volumes logic and use it in the fuzzy compare instead
of the VTK default value. Add a precision calculation from the epsilon so that
any warning messages are printed out with the same precision as was used to
check the values.
Added a sanity check to see if the smallest volume spacing is within 3-10 orders
of magnitude of epsilon and print a warning if not.
Added a python test to exercise the epsilon, precision and geometry check.

TBD: GUI for setting epsilon or just always use a volume spacing based value.