class TGTRA: public TBRIK

 GTRA is a general twisted trapezoid. The faces perpendicular to z are trapezia
 and their centres are not necessarily on a line parallel to the z axis as the
 TRAP; additionally, the faces may be twisted so that none of their edges are
 parallel. It is a TRAP shape, except that it is twisted in the x-y plane as a
 function of z. The parallel sides perpendicular to the z axis are rotated with
 respect to the x axis by an angle TWIST, which is one of the parameters. The
 shape is defined by the eight corners and is assumed to be constructed of
 straight lines joining points on the boundary of the trapezoidal face at z=-DZ
 to the corresponding points on the face at z=DZ. Divisions are not allowed.
 It has 15 parameters:

     - name       name of the shape
     - title      shape's title
     - material  (see TMaterial)
     - dZ         half-length along the z axis
     - theta      polar angle of the line joining the centre of the face
                  at -DZ to the centre of the one at +DZ
     - phi        azimuthal angle of the line joining the centre of
                  the face at -DZ to the centre of the one at +DZ
     - twist      twist angle of the faces parallel to the x-y plane
                  at z = +/- DZ around an axis parallel to z passing
                  through their centre
     - h1         half-length along y of the face at -DZ
     - bl1        half-length along x of the side at -H1 in y of
                  the face at -DZ in z
     - tl1        half-length along x of the side at +H1 in y of the face
                  at -DZ in z
     - alpha1     angle with respect to the y axis from the centre of
                  the side at -H1 in y to the centre of the side at
                  +H1 in y of the face at -DZ in z
     - h2         half-length along y of the face at +DZ
     - bL2        half-length along x of the side at -H2 in y of the face at
                  +DZ in z

     - tl2        half-length along x of the side at +H2 in y of the face
                  at +DZ in z

     - alpha2     angle with respect to the y axis from the centre of the side
                  at -H2 in y to the centre of the side at +H2 in y of the
                  face at +DZ in z