Mjcf : Fix parsing of armature parameters

[MegMll]

This pr aims at correcting parsing of armature parameter available in mjcf models. As of now, mjcf provided only one value per joint, but in pinocchio armature is based on the model.nv.

In this pr, we modify how armature is parsed and use the pinocchio standard.

Another aim of this pr is to add support for default tag that does not have a class name. Until now, this was taken into account in the mjcf parser.

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
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• build_autodiff (build Pinocchio with CppAD support)
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Thanks.
The Pinocchio development team.

[jcarpent]

Very nice Megan ;)