fix edge case in algebraic number comparison

[ValentinPromies]

So far, algebraic_numbers compare_core handles an edge case incorrectly:

• If the two compared numbers (a, b) are different,
• the intervals still overlap after refinements, and
• both a and b are a root of the second polynomial (cell_b->m_p), e.g. they are the first and second root

then the method would return sign_zero (i.e. "equal"). This behavior can be replicated with the provided test case (before the fix). This requires algebraic.factor=false, though i first encountered it during solver runs on QF_NRA instances with the default algebraic.factor=true, which apparently means that the polynomials for anums are still not always factored.

The fix is to compare the interval bounds of b to a and vice versa. Then the Sturm-Tarski check is only run if a and b both lie in the intersection of the intervals, because only then is it guaranteed to be correct.

[NikolajBjorner]

@levnach