adding verbose kwarg
removing min_shells/max_shells property getter/setter decorator methods
updating docstrings
adding deleter decorator methods for n, m, nz attributes
changing vdw_spacing attribute from private to public
Changing structure_atoms to a public attribute.

Removing unnecessary getter/setter decorator methods for
structure_atoms.
Updating unit_cell to a public attribute.
adding deleter decorator methods
Don't cast nx/ny to ints
Bug fix.

Need to set `assume_integer_unit_cells` before setting
`nz` attribute.
Updating attributes and property methods.
Replacing `structure_atoms` attribute name with `atoms`.
Changing `structure_atoms` attribute name to `atoms`.

Other small changes related to private/public attribute
name changes and corresponding property decorator methods.
updating sknano.io tests
Changing `structure_atoms` attribute name to `atoms`.

Other small changes related to private/public attribute
name changes and corresponding property decorator methods.
updating test functions for MWNT generator classes
updating core.atoms test functions
updating MWNT test functions
Changing `chiral_type` values to the full name.

The function `get_Ch_type` now returns the full name spelling
of the chiral class (`armchair`, `zigzag`, or `chiral`)
and checks of chiral type now check that `chiral_type` is one
of (`achiral`, `armchair`, `zigzag`, or `chiral`) instead of
using the abbreviations (`aCh`, `AC`, `ZZ`, `Ch`) which I
think is too ambiguous.
moving import of SWNT class to the function that uses it