Add SymPy support

[bchretien]

For simple functions that can be written as SymPy expressions, we could automatically generate the RobOptim function (impl_compute, impl_gradient, and even impl_hessian) from the SymPy expression. This would be as easy as using finite differences, yet it would be much more efficient. We could even imagine adding sparse matrix support.

[thomas-moulard]

Yes, it is a nice wishlist feature. I hope I had the time to do that now :)

Actually, this part is Python only so as soon as the C/C++ interface is ready then developing the
pure-Python features will probably be straight-forward.

[bchretien]

We could also consider providing support for automatic differentiation which may be even better (e.g. with ad).