Refactoring sknano.io class hierarchy.
Refactoring sknano.io class hierarchy.

Removing StructureReader metaclass.
Changing StructureWriter metaclass to subclass of object.
Removing unnecessary custom exception classes.
Importing Atom/Atoms class which all io subclasses
should import and use.

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