class TGeoPolygon: public TObject

 TGeoPolygon - Arbitrary polygon class


 A polygon is a 2D shape defined by vertices in the XY plane. It is used by
 TGeoXtru class for computing Contains() and Safety(). Only the pointers to
 the actual lists of XY values are used - these are not owned by the class.

 To check if a point in XY plane is contained by a polygon, this is splitted
 into an outscribed convex polygon and the remaining polygons of its subtracton
 from the outscribed one. A point is INSIDE if it is
 contained by the outscribed polygon but NOT by the remaining ones. Since these
 can also be arbitrary polygons at their turn, a tree structure is formed:

  P = Pconvex - (Pconvex-P)           where (-) means 'subtraction'
  Pconvex-P = P1 + P2 + ...           where (+) means 'union'

  *Note that P1, P2, ... do not intersect each other and they are defined
   by subsets of the list of vertices of P. They can be splitted in the same
   way as P*

 Therefore, if C(P) represents the Boolean : 'does P contains a given point?',
 then:

 C(P) = C(Pconvex) .and. not(C(P1) | C(P2) | ...)

 For creating a polygon without TGeoXtru class, one has to call the constructor
 TGeoPolygon(nvert) and then SetXY(Double_t *x, Double_t *y) providing the
 arrays of X and Y vertex positions (defined clockwise) that have to 'live' longer
 than the polygon they will describe. This complication is due to efficiency reasons.
 At the end one has to call the FinishPolygon() method.