class THelix: public TPolyLine3D

 Helix is, hmmm, well, a helix.  It has 3 different constructors.

 Comments/suggestions/etc on this class should be sent to the author:
             pyeh@cdfsga.fnal.gov (Ping Yeh)

   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(x0,y0,z0, 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 initial position
   (x0,y0,z0) and "velocity" (vx0,vy0,vz0), both 3-vectors, and
   an angular frequency w.  The parametric equation of the helix is

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


   The 'normal constructor' has 6 parameters,

       Example:
                 THelix pl1(xyz0, v0, w, range, rtype, axis);

         where:
             xyz0  : array of initial position
             v0    : 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



 A Third constructor uses several default values:

       Example:
   c1 = new TCanvas("c1");
   TView *view = new TView(1);
   view->SetRange(-1,-1,-1,1,1,1);
   THelix *helix = new THelix(0.0, 0.0, 0.0, 1.0, 0.0, 0.3, 10.0);
   helix->Draw();

 will initializes a helix with its axis in Z direction (rtype=kHelixZ).
 range[0] = 0 and range[1] = 1