F remove deprecated parent

Summary

For #94 (closed). Removes the deprecated parent keyword for good.

Focus

Simply removed the special treatment; it now raises a ValueError like any other unknown keyword would.

Test Environment

Unit test is sufficient.

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src/doc/yaml_interface.rst

@@ -109,7 +109,6 @@ Keywords
   added as parents, and all Properties with at least the importance ``XXX`` are inherited.  For
   example, ``inherited_from_recommended`` will inherit all Properties of importance ``recommended``
   and ``obligatory``, but not ``suggested``.
-- **parent**: Parent of this entity. Same as ``inherit_from_obligatory``. (*Deprecated*) 
 
 Usage
 =====