feat: add plothist style

[andrzejnovak]

@0ctagon @cyrraz @jonas-eschle

[jonas-eschle]

Thanks for going forward with this! About the style name, isn't this maybe better to be named BelleII or similar?

[andrzejnovak]

Yes, I would be in favour of naming it "Belle" which would be consistent with the rest of the styles and I assume safe wrt Belle vs Belle2 :) but I'd let @0ctagon and @cyrraz opine on that

[0ctagon]

I think we need:

• The default style of plothist for the examples in the documentation, which we would call plothist for legacy or default, base, masterclass...
• The default Belle II style, which by construction would be very similar to the current plothist style, that we would call Belle2. To push this into mplhep, I think we would need approval from the Belle II publication committee (and they might change the style a bit from plothist). I will look into this.
• I have no idea about the Belle plot publishing guidelines, I will also look into this so we can add it.

[andrzejnovak]

Well, let's put that in for now and we can revisit it / include dedicated Belle style later