Add plot_fluxlike CLI

• Fix the plot subcommand for the qualikiz_tools CLI.
  • Uses native Python argparse instead of the extra package docopt. The idea is to eventually move all CLIs to argparse.
  • Has --help and usage information
  • Adds the now-working sub-subcommand plot_fluxlike, which produces a plot of a 1D scan + relevant spectra
• Replaces the Michele discrete slider by the now-working snapping slider from matplotlib https://matplotlib.org/devdocs/gallery/widgets/slider_snap_demo.html
• Many fixes to the unholy monster of Frankenstein from @MicheleDiffer and @Karel-van-de-Plassche plot_fluxlike.py
  • Fix since-broken usage of xarray and pandas
  • Fix FutureWarnings from pandas
  • Fix multirun plotting
  • Add more axis labels
• Add helper function to generate unique filenames (for figures)
• Add/fix helper functions to generate "pretty names" for QuaLiKiz variables in pretty_names.py

TODO:

• Find out new lower bound on matplotlib
• Add even more axis labels to plot_fluxlike.py
• Always regenerate netCDFs