CLI: Allow plot commands to plot results for multiple work chains

[mbercx]

Currently the plot commands can only plot the results for a single work chain. However, since one of the nice aspects of the common workflows is to be able to easily compare results for different codes, it would be nice if the commands could take multiple work chain identifiers and plot them in the same figure.

In doing this, I would also add an option to add labels (-l/--labels) to the plots, which can then be used for the legend.

I'm not sure how to best adapt the method for when using the --print-table option. Either print separate tables, or one big table where we also use the labels to identify the columns with the results. This might get very bulky, though.