Cache DOS colours and re-use them

[ajjackson]

This addresses #8 by ensuring that orbital colours will be consistent across multiplots and between re-plot calls from an interactive notebook.

A dict sumo.plotting.colour_cache is held at the plotting level (as other plots may wish to share this information) and initialised as {}; when the dos_plotter does not find an orbital colour specified in the colours argument it will take the next item from a cycle and update the cache with {element: {orbital: colour}}. To clear the cache and start the colour cycle from the beginning, this module-level variable can be manually reset to {}. This would rarely be necessary: I still recommend that any real colour tweaking should be done by passing a dict or config directly.

Tests are included and currently pass.

[utf]

Thanks @ajjackson. Just tested this and it works great.