[WIP] Draft of new layout for python modules

[sylvlecl]

Signed-off-by: Sylvain Leclerc [email protected]

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#300

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Refactoring

[sylvlecl]

@geofjamg @EtienneLt @CBiasuzzi @massimo-ferraro @obrix :

please do not hesitate to formulate a critical opinion on this proposed layout (implemented on loadflow module only as an illustration): all actual code goes into impl subpackage, while the public API is simply defined by importing the classes and methods we want to expose.

This should allow to scale better (more python files, basically ...), and to not care too much about namespace pollution in implementation files.

[EtienneLt]

done with https://github.com/powsybl/pypowsybl/pull/641