Add gnuplot support

[razimantv]

Added gnuplot support based on has2k1's gnuplot kernel.

The kernel supports % line magics. These are treated correctly when exporting to script and percent. But when exporting to hydrogen, the line magic remains as a line starting with % so that it is not a valid gnuplot line anymore. But I suppose that is how hydrogen works?

[mwouts]

Hi @razimantv , thank you so much for your PR! Sorry by the way for not responding before, I have been busy with a few other projects recently...

Yes the PR seems alright at first sight. And indeed the hydrogen format is the same as percent except that by default the magic commands are not commented out (because hydrogen executed the code with Jupyter, but I am not sure if that is still much used nowadays).

There are a few errors on the CI, and I think I have fixed them recently on main, would you like to rebase your PR on main ?

[razimantv]

Sorry, a bit new to this. I merged the main branch, would this be enough?

[mwouts]

Yes it should work - let's see what the CI says!

[mwouts]

Awesome, thanks!