III. Tets: Each tetrahedron is tested separately for refinement or derefinement

where V is the volume of the tetrahedron.
Criteria:

     1) Junction: Refine if any of the tets field values straddle c.

     example: For c = 0 refine if f changes sign between any of the four nodes.

     2) Constant: Refine if any of the tets field values exceed c.
         Tag for refinement if

f(1) > c or f(2) > c or f(3) > c or f(4) > c

     3) Maxsize: Refine if the tet volume exceeds c.
         Tag if V > c

     4) Aspect Ratio: Refine if the tets aspect ratio is less than c. For the tet the aspect ratio (AR) is defined as the ratio of the radius of the inscribed sphere
         of the tet to the radius of the circumscribed sphere. We renormalize this ratio by multiplying by three so that the ratio equals one for a regular
         tetrahedron (composed  of equilateral triangular faces).

 AR = 3 RIN/ROUT  where

RIN radius of inscribed sphere
ROUT radius of circumscribed sphere
AR is never greater than one.

          Tag if AR < c.
          Generally the smaller AR is, the more elongated the tet is.

     5) Lambda Refine: Refine if lamda/dx > c . Where lambda is taken to be the radius of the circumscribed sphere ROUT of the tet.

lambda = |f(xcen)| /grad f
f(xcen) = (f(1) + f(2) + f(3) + f(4))/4

 where xcen is the centroid of the tet, and we have assumed linear interpolations of f.  grad f is evaluated for the tet by an approximation involving linear  interpolation au the surface integral  over the surface of the tet.