Add support for overlaying spectra

[suzil]

Screen.Recording.2022-08-27.at.5.10.03.PM.mov

(The missing ylabel has been fixed, don't want to re-demo it)

I've changed the UI slightly from the ticket to instead have:

• Create: plots the given spectra, replacing the existing plot
• Add: Adds a new spectra

Would a different UI be better?

I hope it's OK @arunavabasu-03 that I took this ticket 🙈

Closes #452

[erwanp]

@minouHub you'll love it

[suzil]

There's some issue with the species I still need to figure out (you'll notice duplicated labels on the legend), just noticed it

[erwanp]

This is a very game-changing feature ! From a physics point of view, it allows very interesting thing :

• compare databases ! (therefore, maybe the database should be in the legend? We might be able to re-use @arunavabasu-03 's filename [Database, Molecule, Temperature, Pressure, etc. ] )
• also, compare interaction of spectra ; i.e. ; compute CO spectrum (1), compute a CO2 spectrum (2), and compare with the overlap of a CO+CO2 spectrum (3)

I like the Add to Plot syntax rather than the originally suggested Clear button. I'd keep that. Maybe we could rename "Create Plot" to "New Plot" ?

[suzil]

@erwanp how is this?

[suzil]

Fixed unit of pressure from atm to bar ^

[erwanp]

Nice and clear ! Can we add one comma after the database name in the label, not to read "Hitran Pressure" as one block?

[suzil]

@erwanp updated to add that comma, anything else?

[erwanp]

LGTM