[1] Implement scale propagation chain

The scale propagation is imagined to work as follows:

[x] Specify the scale you want to start at
[x] Specify the scale (or NME; later) you want to end with
[x] Let the DB figure out and present the intermediate expansion schemes (branches).
[x] Let the user select which branch to take

While (1) and (2) are straight forward, (3) is the key part of the implementation.

Scale Start ----- Scale21 ---------------------- Scale End   | Branch 1
            ---------------------- Scale22 -----             | Branch 2

Ideas which are valuable for usage.

[x] Details about scale maps (how many operators, orders, etc)
[x] If one part of the mapping (partial branch) is unique, automatically pick this one

ckoerber / strops