Comparison of implementations

[mforets]

.. I know it's a vaguely defined issue, but it would be nice to add some text (say, centralized in the MultivariatePolynomial docs with a comparison of the different implementations of the MP interface. We could also add a comparison with other packages such as TaylorSeries.jl. Also comparing with libraries in other languages would be relevant (for potential users of the tool, etc). All this takes a LOT of effort to do properly though.