Fix targetClass not contained in application profile

• some application profiles do not contain the targetclass property
• all application profile contain the sh:NodeShape property
• Therefore I replace the targetClass query with the nodeshape query in the graph.target() method
• Additionally I've added a parameter to the FileObject.form() method to force the client to fetch metadata for the file whenever it creates a form for it. By default this is disabled (set to false). For some profiles the metadata served by Coscine with the resource in JSON-LD format is atm broken. So until there is a fix for that, this provides a nice and easy workaround. Just use object.form(True) instead of regular old object.form() when you are impacted by the issue.