remove_equalities: use Polyhedron_Preimage instead of Polyhedron_Image

remove_equalities: use Polyhedron_Preimage instead of Polyhedron_Image

Polyhedron_Image first computes an explicit representation, performs
the transformation and then convert the resulting explicit representation
back to an implicit representation.
In most practical applications, the number constraints is much
smaller than the number of vertices.  It's therefore better
to use Polyhedron_Preimage instead.

This solves a problem reported by S. P. Tsarev <sptsarev@mail.ru>
where barvinok would get stuck while projecting out the equalities
in a given counting problem.