No mstate in scene is forbidden

[bakpaul]

If no mstate is found then an error is thrown in write mode, and a failure in compare.

I did'nt touch the legacy code because it is legacy, but ideally it shouldn't allow empty mstate list neither.

[fredroy]

This PR is raising an issue for Regression for me ; what if the regression scene just contains a VisualModel, or an other state (if one day there will be other meaningful State?)
Maybe we should consider the broader State than the Mechanical Object

[bakpaul]

You are right, and this would also require a bit more info than the id of the state for the reference file. I have work on adding meta data to reference files, I think that adding the unique identifier (aka its full path in the scene) is a must have to enable using any quind of state.