APF - Implement revised APF-RDW from Messinger et al

[bjrtx]

Source is Effects of Tracking Area Shape and Size on Artificial Potential Field Redirected Walking, by Messinger, Hodgson and Bachmann (2019).

[bjrtx]

Basic ideas:

• obstacles are broken into smaller segments
• each segment's contribution is proportional to its length (i.e. surfacic repulsive charge)
• variable fall-off exponent
• there seems to be some confusion in the literature between potentials and forces

[BenoitGeslain]

Basic surface APF implementation in ddb839a. Possible optimisation is to use the surface of the triangle created by the user and the segments points?