Add imshow flexibility

[zkbt]

The changes include:

• Add an xaxis='time' or xaxis='wavelength' option to change the orientation on the imshow plots.
• Add _add_panel_labels helper to add (a), (b), (c) labels to multipanel plots. Apply this to .imshow_data_with_models().
• In .plot_quantities, change the data_like keyword to xaxis for consistency with .imshow and .plot.
• A few tiny bug fixes to plot_spectra that came up while testing on new examples.