Renaming SpringSliderDashpot to PairSpringSliderDashpot to allow a

WallSpringSliderDashpot model.

Correctly identifying non-flat - sharp - wall interactions, i.e. with
concave points and edges.  Rigoruous methods for exclusion distances
of existing wall interaction sites.

Separated off wall force calculation into WallSpringSliderDashpot
model.