removing empty defect_analyzer module
updating sknano.utils setup.py
renaming _funcs to _extras
updating __init__ imports
replacing plural_word_check with pluralize
adding total_ordering methods for custom object comparisons
updating docstring module name
adding total_ordering methods for custom comparison of Point objects
updating __eq__ method and adding property for unit_vector.

__eq__ method now checks the equality of the coordinates and the
head and tail Points of the Vector.
updating tests
separating the structure classes into separate modules for each class
updating geometric_structure code
raise NotImplementedError vs return it