Calculate footprint centroid based on its pads.

[dzid26]

Solves issue #379 that silkscreen was affecting centroid position calculation.

Nice and center now regardless of the uneven silk:

[Bouni]

I completely overlooked this PR, soory 🥵

Have you tested this with V7 and V7.99?

[dzid26]

I did.
The one thing you may want to consider is whether this method of calculation of the center should be used for all footprints (smd, thruhole, and undefined), or just undefined (which is what I've got after importing footprints from altium).
#379 (comment)
In the end, I decided it would use bounding boxes for all of them.

[Bouni]

Please rebase and fix the linter issue (ruff --fix fabrication.py)