BUG: polynomialEqns: fix discriminant limit

It was observed in a MPPICDyMFoam simulation involving a single particle
in a moving mesh that the barocentric trajectory of the particle follows
an unexpected path at some arbitrary instant in time.

The issue was tracked to "hitEqn()" where cubicEqn/quadraticEqn computes
one of the roots wrongly due to the discriminant limit we set, e.g. for:

    0x^3 + 1.4334549e-33 x^2 - 9.0869006e-10 x + 0.0027666538

Although the discriminant limit was carefully selected to avoid various
problems at the time, the new change is required more due to its exposition
to a wider spectrum of applications.