Module drawing

source code

All the routines to choose the position to each vertex and the 
direction for particles. All those class are not related to any output class.

This file contains 4 class:
    * FeynmanLine which extend the Leg with positioning information
    * VertexPoint which extend the vertex with position and line information.
        Coordinates belongs to [0,1] interval
    * FeynmanDiagram which
            1) Extends a diagram to have position information - load_diagram 
            2) Is able to structure the vertex in level - define_level 
                level are the number of s_channel line-initial particles
                separating the vertex from the initial particles starting point.
            3) Attributes position to each vertex - find_initial_vertex_position
    * FeynmanDiagramHorizontal
        is a child of FeynmanDiagram which assign position in a different way.

    The x-coordinate will proportional to the level, both in FeynmanDiagram and 
        in FeynmanDiagramHorizontal
    
    In FeynmanDiagram, the y-coordinate of external particles are put (if 
        possible and if option authorizes) to 0,1. all other y-coordinate are 
        assign such that the distance between two neighbor of the same level    
        are always the same. 
    
    In FeynmanDiagramHorizontal, an additional rules apply: if only one 
        S-channel is going from level to the next, then this S-channel should be
        horizontal.