Bands/multiweights:

• Draw multiweights on the canvas
• Option to combine PDF multiweights and draw on canvas as a band, by supplying BandComponentPDF option (c.f. https://gitlab.com/hepcedar/rivet/-/blob/release-3-1-x/doc/tutorials/multiweights_plotting.md#pdf-bands)
  • Using LHAPDF tools in Rivet, then pass to YODA 

TODO - before 3.1.8 release

• Tidy-up code
• Rebase
• Merge request matplotlib-3-1-x  into release-3-1-x, 
  • with rivet-mkhtml-mpl 

TODO - after 3.1.8 release

• Combine weight by working out envelope (similar to https://gitlab.com/hepcedar/rivet/-/blob/release-3-1-x/doc/tutorials/multiweights_plotting.md#envelopes)
• Integration in Contur:
  • Currently, need to run contur before contur-mkhtml as the CLs values are printed on the canvas, contur-mkhtml uses .dat files written out when running contur
  • Potentially reshuffle so that you can run contur-mkhtml on the "raw" YODA-files, fetch CLs and additionally draw on canvas if they are available (i.e. contur has been run before)