Handle conversion from Eigen::Matrix to numpy.array or numpy.matrix

Fix wrong message error

This follows issue #44.

[CMake] Synchronize module

Add note for compiling eigenpy with python3 on macOS

Getting eigenpy (and pinocchio) to compile on macOS with python3 took me some effort. The required trick turned out to "only" add `-DPYTHON_EXECUTABLE=\`which python3\`` to the cmake argument.

Do you think this is worth adding to the README.md/installation instructions? I am happy to provide something similar for pinocchio as well.

Fix compatibility issue with Eigen 3.3.5

Register Eigen::MatrixBase

Handle zero-size vector

Python 3 tests & CI

cf. https://github.com/travis-ci/travis-ci/issues/5821

Python 3 is tested by gitlab anyways.

Improve compatibility with Python3 and recent Boost versions >= 1.67.0