Added the Python bindings of StateVector + an unit-test for StateAbstract bindings

In this PR I did the following tasks:

1. Improved the method for getting nx and ndx values (StateAbstract class)
2. Added the Python bindings for StateVector
3. Added an unit-test skeleton for StateAbstract bindings
4. Included a code that tests the StateAbstract bindings (with a derived StateVector class in Python) against the StateVector bindings.

@mnaveau I will create as much as possible the unit-test code for Python bindings. This isn't my personal priority but a needed step to check the Python bindings behaviour.

This code is functional (bindings and unit-test), and you can run it by

• enabling the Python bindings generation, and
• running

 make test