Action and states Pythiong bindings + extra improvements

The main purpose of this PR is to integrate Python bindings of the action and state classes. Additionally there are other important modifications:

• solving slightly difference with the Python API.
• renaming the variables to follow Gepetto standard (as @ostasse pointed out early).
• adding Python bindings unit-test.
• formatting the c++ code and Python to follow Gepetto standard.
• changing the installation folder of Python legacy code.
• reorganizing Python bindings code.
• fixing errors with the unit-test.

Important remark

We cannot write internal values of Eigen objects in the Python bindings. I have reported this issue in eigenpy (see https://github.com/stack-of-tasks/eigenpy/issues/65).

I propose to fix it after dealing with the needed changes in eigenpy. I have reported this issue here #179 (closed).