[Core] Warn when input/output of mapping is deduced automatically

[alxbilger]

The search for "input" and "output" can be moved into the init method. It is called anyway by the object factory. If we do that, the function create is identical to BaseObject::create. It is then useless to define it again.

In case a mapping is defined without "input" or "output", the warning looks like:

[WARNING] [RigidMapping()] The attribute 'input' of this component has not been defined. It is automatically set to '@/Torus3/MechanicalObject1'. It is recommended to define it to avoid any error.
[WARNING] [RigidMapping()] The attribute 'output' of this component has not been defined. It is automatically set to '@/Torus3/Surf2/MechanicalObject1'. It is recommended to define it to avoid any error.

To better understand the implicit mechanism, the full link is set. It is much more readable than "@../" and "@./"

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• it builds with SUCCESS for all platforms on the CI.
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• it does not break API compatibility.
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[fredroy]

[ci-build][with-all-tests]