Add "best-fit" equilibrium

[tomvanmele]

Currently, the equilibrium geometry of a form diagram is generated by computing horizontal and vertical equilibrium in two separate steps.

Although this is very educational, it is often difficult to achieve specific results.

For scenarios where a specific outcome is desired, we should a function that computes the "best-fit" equilibrium for a given target geometry.

The corresponding RV workflow button could execute the following steps:

• ask user to select a target
• compute target heights for each of the form diagram vertices
• compute a force distribution that results in a close match of the target heights
• update form and force diagram