[Sofa.Core] Merge BaseContext and Context

[damienmarchal]

Related to the issue: #5556
The PR is based on #5566, please merge it to make the difference clear.

At this step, it is unclear to me how to move forward.
The Context seems to be a not very well defined concept as it shares properties with the scene graph structure.

The fact it stores data field like dt, t, gravity... and it is also a BaseObject makes me think that possible path could be to simply add make it to the factory and have the context as one of the a node component. The get<> should probably be moved out of the context unless we want a context to explore the local children and objects or not.

def createScene(root):
       root.addObject("Context", dt=0.01, gravity=[0.0,9.8,1.8])      

This will breaks everything that makes the assumption that dt, node are node attributes.

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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).

[damienmarchal]

Close because it is now in #6266