class TriMesh: public ZGlass, public TriMeshColorArraySource

 Set mass of the mesh and calculate center of mass and Jz (should
 extend for full matrix but let's keep it simple for now) with a
 simple algorithm: distribute mass_frac_on_mesh of mass over the
 triangles of the mesh.

 As J_spehere = 2/5 m*r^2, the default value for mass_frac_on_mesh = 0.4.

 Estimate XY-area, volume and mass from given parameters and
 bounding-box of tring-tvor.

 Calculates bounding box of the mesh.

 Extrude triangle with index 'ti' and place apex of the new tetra
 at height 'h' above the triangle's cog.

 Extrude triangle with index 'ti' and place apex of the new tetra
 at the given position.

 Colorize the triangles in a standard way.
 Downwards - blue, otherwise - green.
 It is assumed that the triangle normals have been calculated.

 Colorize the triangles with given color.

 l1 ~ fwd length; l2 ~ bck length; w ~ width at the end; h ~ height;

 l1 ~ fwd length; l2 ~ bck length; w ~ width at the end; h ~ height;

 Create a box centered on 0 in x and y directions and going from 0
 to c in z.

 Find world-point corresponding to the passed fgh coordinates and return it
 in the xyz_out array.

 Flag absolute_h determines how the input h-coordinate is interpreted and
 also what will be returned in h_out:
   true  - h is absolute, h_out is relative to the surface;
   false - h is relative to the surface, h_out is absolute h.

 trianlge_idx (optional) is index of the triangle on the surface that was
 hit by the ray used to find the surface point. If you need vertex index as
 well, call 'Int_t FindClosestVertex(xyz_out, triangle_idx)'.

 Returns false if ray-mesh intersection fails.

 Given world-point xyz_in and relative height h_in, find the
 corresponding point xyz_out at specified height. The point is
 only translated in vertical direction. xyz_in and xyz_out can
 point to the same location.

 h_out (if non-null) is set to relative height of the xyz_in point.

 trianlge_idx (optional) is index of the triangle on the surface that was
 hit by the ray used to find the surface point. If you need vertex index as
 well, call 'Int_t FindClosestVertex(xyz_out, triangle_idx)'.

 Returns false if ray-mesh intersection fails.

 Find vertex of triangle that is closest to point given by xyz.
 If sqr_dist is passed, it is filled with square distance of the returned
 vertex.
 Trianlge index is not checked, neither is tvor's presence.

 Given triangle, position xyz and direction dir, find the exit point from
 the triangle.
 If next_triangle is non-null, the triangle bordering the crossed edge is
 returned there.

 There are very many ways this can go wrong ... still working on the
 interface and the return value.
 1. The direction is orthogonal to triangle normal.
 2. All intersection points are either outside of triangle or at negative
    times.
 3. Next triangle index is -1 - the end of the world is hit.

 Now, a hack was added to also succeed when the triangle edge has just
 been passed: the function is re-entered with the new triangle. Again this
 can go wrong, so a set of triangles that have already been tried is
 cross-checked before re-entry to avoid infinite loop.

 Visits vertex by calling VertexVisitor::VisitVertex(vertex). If
 this returns true, the neighbouring vertices are visited in order
 specified in the vertex edge-array.

 Set visited_vertices holds already visited vertices, they will
 not be visited twice.

 Set accepted_vertices holds the vertices for which the visitor
 returned true.

 Returns the maximum depth of recursion.