Fixes T75032.

• implemented the algortihm that would merge vertices to the weighted center between them
• exposed the merge threshold to the user

I didn't implement the versioning code yet, because I didn't really figure out yet how it works in blender.
Versioning could/should be added to this Diff before commit, but even if it's not added it wont break existing files,
but existing files might look a little different if degenerate geometry was present since the tolerance was
previously set to 0.0003 and without versioning it will now be 3.7e-23 if I'm not mistaking.

The new default tolerance is 0.0001

when testing evaluate the step size for the slider, because I'm unsure about that.
We should really have logarithmic sliders for these kind of values that are always bigger than 0.