Create simple ZMP estimator

Since the control (handled by me) and the estimation (handled by @fbailly) are being worked on in parallel, a quick-and-dirty way to "measure" the ZMP is needed in order to test the control schemes before the full-blown estimator is ready. Up to now, I was using the dynamic.zmp signal, but I've just learnt from @ostasse that this signal is meaningless on Talos. Therefore, I will build a simple entity that trivially computes the ZMP from the plain formulas, with no assurance of accuracy or real-word applicability. I've talked with @fbailly, he agrees and thinks it could be useful for the future too. If needed, we can improve it later.