Implement arc! primitive

[jpthiele]

This adds a new primitive for mesh construction.
With it we can now construct a set of faces whose points lie on a circle around the parameter center.

Important note:
This still needs some more testing and especially automated tests.
It works for the 'Plate with a hole', but there the center is the origin and both points lie in the first quadrant.

I'll try to work on some tests tomorrow, so as a note to me (and maybe someone else) I would test

• That an error is correctly thrown if start- and endpoint are not on the same circle around the center
• That it correctly constructs arcs with points in all four quadrants w.r.t. the center
• That it correclty constructs a large counter-clockwise arc when the endpoint is 'in front' of the starpoint when seen in between [0,2pi]
• That it correctly constructs the arc for a nonzero center point

[jpthiele]

Btw: I have found an error incircle! sin and cos where the wrong way around.
I fixed that in this PR since I added a test to ensure both produce the same points.

@j-fu FYI: If all tests pass now this PR is ready from my side

[j-fu]

Looks good to me now. May be add a statement to the changelog.

[jpthiele]

Looks good to me now. May be add a statement to the changelog.

Check!

And I rebased on current master so the changelog is up to date.