// example on using with exteral classes (doxygen page)  


/**
 
 \page ExtUsagePage Examples with external Packages


<h3>Connection to Linear Algebra classes</h3>

It is possible to use the Vector and Rotation classes together with the Linear Algebra classes. 
It is possible to  set and get the contents of any 3D or 4D Vector from a Linear Agebra Vector class 
which implements an iterator or something which behaves like an iterator. For example a pointer to a C array 
(double *) behaves like an iterator. 
It is then assumed that the coordinates, like (x,y,z) will be stored contiguosly.  

<pre>
TVectorD       r2(N)                        // %ROOT Linear Algebra Vector containing many vectors
XYZVector      v2; 
v2.SetCoordinates(&r2[INDEX],&r2[index]+3); // construct vector from x=r[INDEX], y=r[INDEX+1], z=r[INDEX+2]
</pre>
To fill a Linear Algebra Vector from a 3D or 4D Vector, with GetCoordinates() one can get the internal coordinate data.
<pre>
HepVector      c(3);                        // CLHEP Linear algebra vector
v2.GetCoordinates(&c[0],&c[index]+3 )       // fill HepVector c with c[0] = x, c[1] = y , c[2] = z
</pre>
Or using TVectorD
<pre>
double * data[3];
v2.GetCoordinates(data,data+3);        
TVectorD       r1(3,data);                  // create a new Linear Algebra vector copying the data
</pre> 
In the case of transformations, constructor and method to set/get components exist with Linear Algebra matrices. 
The requisite is that the matrix data are stored, for example in the cse of a LorentzRotation, from (0,0) thru (3,3)
<pre>
TMatrixD(4,4) m;
LorentzRotation r(m)                        // create LorentzRotation from matrix m  
r.GetComponents(m)                          // fill matrix m with LorentzRotation components
</pre>

<h3>Connection to Other Vector classes</h3>

The  3D and 4D vectors of the GenVector package can be constructed and assigned from any Vector, which satisfies the following requisites: 
<ul>
<li> for 3D Vectors and Points must implement the x(), y() ans z() methods
<li> for LorentzVectors must implement the x(), y(), z() and t() methods. 
</ul>
 
<pre> 
CLHEP::Hep3Vector hv; 
XYZVector  v1(hv);                          //  create  3D Vector from  CLHEP 3D Vector

HepGeom::Point3D<double>  hp; 
XYZPoint p1(hp);                            // create a 3D Point from CLHEP geom Point

CLHEP::HepLorentzVector  hq; 
XYZTVector    q(hq);                        // create a LorentzVector  from CLHEP L.V.
</pre>


*/