Improve differentials performance.

Performance of differentials is improved by casting the
polynomials involved in modular reduction to a
computationally efficient ring.
Rewrite RiemannSurface.cycles()

Now that homology returns a- and b- cycles. Rewrite
appropriate parts of RiemannSurface to account
for this new output format.
Fix 3d plotting in RiemannSurfacePath.

When Smale's alpha theory was applied in #a7c7cd57 the
output of RiemannSurfacePath.analytically_continue() changed
to a sheet tuple. RiemannSurfacePath.plod3d() needed to be
updated with the new output format.
Checkpoint rm.