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 it
is tagged and then during the sorting of complexes if that
tag or the Poly associated with an intervale is a quadratic
binomial then 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 during sorting -- I don't
know why the tagging is not working for that case.