Refactor required odML attribute checks in parsers

[mpsonntag]

• remove any hardcoded checks in any of the parsers.
• solely depend on the the required attributes defined via format.py in all of the parsers.

[mpsonntag]

These changes lead to a change in Section and Property.__init__ behavior: name will become optional to make documents load- and parsable which contain unnamed Sections or Properties.

To make sure that an odML document does not contain unnamed Sections or Properties, the value of id will be used as name attribute value, if name was not provided when instantiating.