Concept for the positioning of nodes

As stated in #94 (closed), it is not clear if the nodes should be positioned in a coordinate system which defines the optical axes, or if the optical axis is defined in the first place and the nodes are attached to it.

For the latter, it would be useful to create standard anchor points of optics on an optical axis, such as the center of a lens or a mirror surface.

From this anchor point, addtional manipulation of the orientation (misalignment) may be defined, e.g. tip, tilt, roation, translation... This may be used for a stability analysis of an optical setup