[examples] Make scene resolution independent + doc

[alxbilger]

The MechanicalMatrixMapper example scene deserved a bit of love.

• Converted to Python3
• Add documentation (the concept of rigidification deserves a more detailed doc somewhere else)
• It is possible to change the resolution of the grid while keeping the same behavior: 1) indexPairs is built algorithmically, 2) ConstantForceField is applied using a BoxROI

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

As it is using SofaPython3 to run, should not it be ... in the SofaPython3 plugin 😶 ?

[alxbilger]

For the record, the scene was already here before this PR, and it required Python...

There are pros and cons about putting Python scenes in SOFA. Let's discuss it.

For this scene in particular, Python is required as the scene has a Node with two parent Nodes. It also allows to compute indexPairs.

[fredroy]

@alxbilger I understand your point, especially because it is the only scene showing an example on how to use MechanicalMatrixMapper...