Unify Section.__init__ and Sectionable.create_section behavior #368

The base.Sectionable class features a create_section method that allows the creation of a new Section from a Document or a Section and sets the new Section as the child of the calling parent.

The methods' arguments currently only include name, type and oid and features a different behaviour than the Sections own __init__ method:

• name cannot be empty ; this should be changed to empty being default and allowed
• type is by default set to undefined; this should also be changed to empty being default and allowed
• all other attributes one can assign via Section.__init__ should also be assignable via the create_section method with the exception of parent.

When these changes are implemented, the section_type_undefined odml validation should be refactored: the check for string undefined should be removed and the warning message rephrased, since the above described behaviour is the only occurrence of setting the Section.type to undefined.