Better handling/plotting of multiple Spectrum2D

[rebeccafair]

This has arisen as a result of discussion in #155 (for #116). Multiple Spectrum2D objects (e.g. for multiple PDOS maps) are currently just returned as lists, and plotted manually with plot_2d_to_axis. It may be useful to provide a function to automatically plot multiple Spectrum2D, depending on the ongoing use of PDOS maps and if there are any other use cases.

[ajjackson]

As commented on #155 I am becoming convinced that a Spectrum2DCollection would be useful if it had metadata search/split/combine capabilities. In which case we could update plot_2d to work very analogously to plot_1d, with a list of Spectrum2DCollections being tiled left-to-right by list elements and top-to-bottom by the contained spectra.

(And, presumably, blowing up if the metadata don't match...)