rc entry for legend box linewidth

[anirudhkrishnan2718]

Hi,

In matplotlib, setting 'patch.linewidth' can be used to change the legend box linewidth indirectly.
Add a direct rc entry to set this, in the same vein as legend.geo.linewidth.

[cvanelteren]

You can control rc parametrs from matplotlib directly, but after a brief test I see that for some reason patch.linewidth is not responding on line plots but seems to be working for other artists.

[cvanelteren]

In short, all matplotlib rcs are also ultraplot rcs so we are backwards compatible. We just add to matplotlib in a meaningful manner.

[cvanelteren]

Ok so I looked in the wrong direction. patch.linewidth would control the linewidth for all 2D artists. I think adding these for legends is great.

We could add a general legend.linewidth to control the frame width.

[cvanelteren]

One would need to modify _parse_frame() and add the rc to the rcsetup.py

[cvanelteren]

@K-Mirembe-Merc how bout this^