Kromer plot modification for clarity

[jamesgillanders]

When using the nelements flag in the kromer plots, it is possible to wind up excluding elements that make a contribution to the overall spectrum. Presently, this comes with a warning in the docs. However, it would be good to adopt some variant of the modification introduced with the species_list feature, where any contribution from species/elements not included in the colourbar show up on the plot, but in grey. This should be relatively easy to introduce, once the PR on the species_list feature is merged