Refactor system description

This is the first part of the refactoring of the SystemDescription and its Store objects. It moves loading and saving of descriptions to the SystemDescription class and makes sure that there always is a Store object associated with a SystemDescription.

The next steps is to move the code handling the file store of a description to the ScopeFileStore class.

After that there still is some cleanup to be done and to be reviewed that the responsibilities of the classes are met.

Perceived complexity for from_json is too high. [8/7]