build:ShoulderCase: Give the access to ShoulderCase constructor to ShoulderCaseLoader only.

If a ShoulderCase is measured on one system (i.e lbovenus) and then loaded on another one (i.e local system) the saved paths will be wrong.
To avoid using these invalid paths and messing with systems' folder tree, a ShoulderCase can be instanciated by a ShoulderCaseLoader only.

Moreover, some class now have a propagateDataPath method to help keep these paths updated.