B/W mode for plots

[albireox]

Many of the plots that Marvin generates (or will generate) are close to publication quality. One of the frequent needs for publications is to provide a B/W version of the plot because it seems it's still 1960.

We could add a keyword to each plotting tool (or try to abstract it somehow) that will produce a paper, B/W, version of the plot in which all the features are reasonably clear (e.g., using dashed lines instead of colours).

[bretthandrews]

For the map plotting, the user can just pass in cmap='gray'.