class THelix: public TPolyLine3D

 THelix has two different constructors.

   If a particle with charge q passes through a point (x,y,z)
   with momentum (px,py,pz) with magnetic field B along an axis (nx,ny,nz),
   this helix can be constrcuted like

      THelix p(x,y,z, px,py,pz, q*B, nx,ny,nz);

     (nx,ny,nz) defaults to (0,0,1).

   A helix in its own frame can be defined with a pivotal point
   (x0,y0,z0), the velocity at that point (vx0,vy0,vz0), and
   an angular frequency w.  Combining vx0 and vy0 to a transverse
   velocity vt0 one can parametrize the helix as

    x(t) = x0 - vt0 / w * sin(-w * t + phi0)
    y(t) = y0 + vt0 / w * cos(-w * t + phi0)
    z(t) = z0 + vz0 * t


   The second constructor has 6 parameters,

       Example:
                 THelix pl1(xyz, v, w, range, rtype, axis);

         where:
             xyz  : array of initial position
             v    : array of initial velocity
             w    : angular frequency
             range: helix range
             rtype: kHelixZ specifies allowed drawing range in helix Z direction, i.e., along B field.
                    kLabZ specifies drawing range in lab frame.
                    kHelixX, kHelixY, kLabX, kLabY, kUnchanged ... etc can also be specified
             axis : helix axis



 Example constructing a helix with several default values and drawing it:




 This initializes a helix with its axis in Z direction (rtype=kHelixZ).