Typo in `EigenBaseVisitor`

[ManifoldFR]

When discussing Simple-Robotics/nanoeigenpy#1 with @Lucas-Haubert, we've realized there is a typo in the EigenBaseVisitor, just here:

eigenpy/include/eigenpy/eigen/EigenBase.hpp

Line 21 in 7102d83

.def("size", &Derived::rows, bp::arg("self"),

I think this should be Derived::size here, and fixing this will probably break the build in multiple sparse solver classes, because several of them indeed do not inherit from EigenBase<T>.