8.2.4.16} Generic tetrahedral. Code: TET

      A TET is a generic tetrahedral (4 faces and 4 verteces).
      Each  TET is defined by 12 parameters:
      Va_x, Va_y, Va_z, (coordinates of the first  vertex)
      Vb_x, Vb_y, Vb_z, (coordinates of the second vertex)
      Vc_x, Vc_y, Vc_z, (coordinates of the third  vertex)
      Vd_x, Vd_y, Vd_z, (coordinates of the fourth vertex)
      The order of the verteces must be such that:
      - The first  face is described by Va, Vb, Vc, which must
        appear counterclockwise when seen in this order from outside
        the tetrahedral
      - The second face is described by Va, Vc, Vd, which must
        appear counterclockwise when seen in this order from outside
        the tetrahedral
      - The third  face is described by Va, Vd, Vb, which must
        appear counterclockwise when seen in this order from outside
        the tetrahedral
      - The fourth face is described by Vb, Vd, Vc, which must
        appear counterclockwise when seen in this order from outside
        the tetrahedral
      Plotting the tetrahedral on a piece of paper, one can see how the
      vertex numbering described above, is much simpler than the description
      in words.

      Example
      TET BdTetr    0.0  0.0  1.0
                    0.0  0.0  0.0
                    1.0  0.0  0.0
                    0.0  1.0  0.0


                                NOTE
 +---------------------------------------------------------------------+
 | Whenever it is possible, the following bodies should be preferred:  |
 |     PLA, PYX, PYY, PYZ, QUA, RPP, SPH, TET, XCC, XEC, XYP, XZP,     |
 |                    YCC, YEC, YZP, ZCC, ZEC                          |
 | These should indeed make tracking faster, since for them extra      |
 | coding ensures that unnecessary boundary intersection calculations  |
 | are avoided when the length of the next step is smaller than the    |
 | distance to any boundary of the current region.                     |
 +---------------------------------------------------------------------+

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