========================================
Generalisation of code for a default geometry extension in the form of a template.
Default geometry extensions provide a standard interface for an extension having a single stored value (e.g. one int or one string).
Python implementation must be provided separatedly.
Due to problems onthe windows platform the virtual inheritance approach must be dropped. NExt to the already reimplemented proeprty interface the Type interface is reimplemented too. This change allows to revert some earlier changes.
