Separate generic cpp part from cpp python part.

[obrix]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

What kind of change does this PR introduce?

To prepare for new usage of graalvm related code to other project a first step is to separate in a generic library all the generic code not explicitly linked to python. This PR try remove the dependency to pybind11 from the part that should remain generic and build a new generic library that can be reused in new project.

What is the current behavior?

A single library with dependency to pybind11 and python.

What is the new behavior (if this is a feature change)?

Two libraries : a generic powsybl-cpp and a python specific pypowsybl.

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [ ] No

If yes, please check if the following requirements are fulfilled

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

[geofjamg]

@obrix is it still rebasable ?

[obrix]

@geofjamg To sum up what remains to be done in another PR :

• Rename powsybl-cpp to powsybl-native
• Rename pypowsybl-cpp to pypowsybl-native
• Change name of the namespace in powsybl-native to "powsybl" instead of "pypowsybl"