Give CylinderSegment a "barycenter" attribute

[OrtnerMichael]

The problem is that the position attribute of the CylinderSegment is always in the center of the full cylinder.

import magpylib as magpy
s = magpy.magnet.CylinderSegment((0,0,1000), (1,2,1,0,20))
s.show(markers=[s.position], style_marker_symbol='o', style_marker_size=10)

When computing interactions or forces or torques we need the geometric center (should also be the start point of the magnetization arrow). Should we add a method s.barycenter() to this class ? Would make these computations much simpler.

[Alexboiboi]

closed by #484