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.
Use the CTK significant decimals utility method to calculate the
precision with which to display differences in compared volumes.
Use the CTK order of magnitude utility method to compare the
spacing of the volumes with the current epsilon being used to
check them.
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.