Found conceptual problem in handling of contact: the constraint-space velocities caused by (unconstrained) applied forces need to be kept separate from the constraint-space velocities due to the current state, because only the latter should be used to classify frictional contacts as sliding or rolling.
- Modified ImpulseSolver abstraction to handle applied forces separately, and updated PGS and PLUS to implement them.
- Modified SemiExplicitEulerTimeStepper to keep the applied forces apart.
- Fixed up some comments and obscure variables names.
- Changed sign on verrExpand to match other verr's and the documentation.
- Added Block model to RigidContact1 to reproduce PLUS problem observed on GazeboReactionForceWithAppliedForce test case.

PLUS now seems to be working properly on a Gazebo test case for which it was failing before.
This code needs some refactoring to recover from the many repairs I've done to it!