Bug fix to fix length of MWNT shells.
updating docstrings
adding `imax` kwarg
updating test code
updating docstrings
fix __str__ method
cleaning up old code and removing `generate_unit_cell` method
moving class methods to NanotubeBundleMixin class
adding new methods to MWNTMixin and NanotubeBundleMixin classes
moving methods to MWNTMixin class
moving methods to SWNTMixin class
tidying up code
updating docstrings and tidying up
fix bug in generate_bundle_coords method
handle deprecated constructor param `Ch`
updating test_mwnt module
fix attribute name
update test_mwnt_generator test module
Removing `fix_Lz` kwarg from `MWNT` constructor.
Small updates/bug fixes to MWNTMixin and NanotubeBundleMixin.