[Python] Set the current cwd to the scene's path

[fredroy]

Current Status

1️⃣ lots of calls to load resources (like MeshOBJLoader, image, etc) using FileRepository::findFiles() need the Current Working Directory (CWD) to be set to the scene's (see sofa-framework/sofa#6135 )

2️⃣ with runSofa, this is set:

• in XML: through SceneLoaderXML :
  https://github.com/sofa-framework/sofa/blob/bc64d21b6961e2f80af0f0b43d8f46cd1249f618/Sofa/framework/Simulation/Common/src/sofa/simulation/common/SceneLoaderXML.cpp#L114

• in Python3: with SceneLoaderPY3:
  

  SofaPython3/Plugin/src/SofaPython3/SceneLoaderPY3.cpp

  Line 112 in e6fb484

  SetDirectory localDir(filename);

Problem

When running directly with python (the process), the CWD is never changed and is set from where you launch it.
So it means that if you do, with a scene using relative paths:

• cd ${HOME} ; python3 <dir_of_your_scene>/blabla.py it wont work
• cd ${dir_of_your_scene} ; python3 blabla.py it will work

which is... problematic in my opinion.

Fix

So this PR sets the CWD from the directory of the script when doing import Sofa

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NB1: I dont really like that things in the scene depends of the CWD anyway (but I wont obviously not change anything in the Sofa code base 🥴)
NB2: SetDirectory is such a bad name for the CWD changer in SOFA (plus it is doing other useful filesystem things and not really related to CWD)

[hugtalbot]

we should avoid changing the current working directory. The path of the scene file should be added to the list of path. This PR is too breaking and we should merge the PR in SOFA