flake8 found undefined name 'f' in struct.py

[paulmueller]

It is unclear to me how the correct code should look like.

igor2/struct.py:204:24: F821 undefined name 'f'
igor2/struct.py:206:65: F821 undefined name 'f'
igor2/struct.py:208:31: F821 undefined name 'f'

[paulmueller]

See commit above, we now raise a NotImplementedError if somebody reaches that part of the code.