ADDED: matplotlib_utilities module

- Moved functions that enhance matplotlib (colormaps, normalizations
etc.) to new module. Module is imported with try-except statement since
functions require matplotlib which may or may not be present.

- Updated wigner_cmap to allow for custom colors that can be defined in
any format acceptable to the Matplotib.colors.ColorConverter.
these lines should be within a try block instead of in `__init__.py`