BF: fix invalid call to parent in minc classes

It was possible to call `data_from_fileobj` and `data_to_fileobj` from a
generic Header object, and the MINC classes use that object, so these
could cause confusion by making invalid calls.  Raise
NotImplementedError to reduce potential confusion.
Merge pull request #226 from matthew-brett/remove-minc-header-data-fetch

MRG: fix invalid call to parent in minc classes

Prevent invalid call to the minc header-like class.