8296: fix nroots ordering
8285,8289: further refine complex intervals while sorting

In order to be able to handle pure imaginary roots, they
have to be identifiable. So when a binomial factor is
identified either at creation of the COmplexInterval or
during the sorting of complexes it is separated from the
other complex intervals.

TODO: find the expression in test_nth_linear_constant_coeff_homogeneous
that fails if imag only checks u.is_imaginary.