[PATCH 3/3] Stronger normalisation method for powers.

Now normalisation method try to express powers with the same base
as monomials of some dummy variables. This allows to make cancellations
of the sort:
(x-1)/(sqrt(x)-1)  ->  sqrt(x)+1

Signed-off-by: Vladimir V. Kisil <V.Kisilv@leeds.ac.uk>