Implementation of linear cell complex.

This package implements linear cell complexes, objects in d-dimension
with linear geometry. The combinatorial part of object is described by
combinatorial maps, representing all the cells of the object plus the
incidence and adjacency relations between cells. Geometry is added on
combinatorial map simply by associating a Point_p to each vertex of
the map.

Taking a 2D combinatorial map, and using 3D points, gives a
Linear_cell_complex equivalent to a Polyhedron_3.
