Avoid user assigning unimplemented fields on containers. (#102)

Avoid user assigning unimplemented fields on containers. (#102)

Before:

    doc.trade.settlement.payment_means.type_code = "30"

was allowed but had no effect as payemnt_means is a container.

Now it gives:

    Traceback (most recent call last):
      File "/home/mdk/src/python-drafthorse/test.py", line 54, in <module>
        doc.trade.settlement.payment_means.type_code = "30"  # Virement
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    AttributeError: 'Container' object has no attribute 'type_code' and no __dict__ for setting new attributes