Fix inaccurate root finding of resultant.
Fix inaccurate root finding of resultant.

The resultant often has roots of large degree that cannot
be resolved accurately by sympy. Using res.nroots(),
artificial (non-existent) roots appear from roots with
high multiplicity. Using RoofOf constructs, some roots
are missed, perhaps due to some error where RootOf labels
a given root as both root i and root j.

The current solution is to first factor the resultant and
then compute the numerical roots of each factor.

cswiercz/abelfunctions,abelfunctions/abelfunctions,abelfunctions/abelfunctions,cswiercz/abelfunctions,cswiercz/abelfunctions,abelfunctions/abelfunctions