changing superclasses of MWNTGenerator.
changing superclasses of MWNTGenerator.

MWNTGenerator is no longer a subclass of SWNTGenerator and has
new/updated methods to generate_unit_cell and generate_structure_data.
This helps make the subclassing consistent with SWNTGenerator and
UnrolledSWNTGenerator classes as well as simplies the MRO

androomerrill/scikit-nano,scikit-nano/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano