Incidence Geometries

New Features:

• The function IsGraph(D) permits to check if the incidence geometry D is a graph.
• The function Graph(D) permits to convert the incidence geometry D into an object of type Graph provided D is a rank two geometry such that all elements of one type are incident with exactly two elements of the other.
• The function Shadow(D, I, F) computes the I-shadow of the flag F in the incidence geometry D.
• The ShadowSpace(D, I) function computes the shadow space of type I of the incidence geometry D.