implement friction anchors, position friction correction, disabled by default. Use colObj->setCollisionFlag(flag | CF_HAS_FRICTION_ANCHOR); See test/RobotClientAPI/SlopeFrictionMain.cpp. In URDF or SDF, add <friction_anchor/> in <contact> section of <link> to enable.
PhysicsServer: properly restore old activation state after releasing picked object
btMultiBodyConstraintSolver: disable flip/flop of contact/friction constraint solving by default (it breaks some internal flaky unit tests)
added test urdf files for minitaur with all fixed joints, or fixed knees.
added some stiffness/damping to minitaur legs (testing)
tiny_obj_loader, don't crash on invalid texture coordinates
btMultiBodyConstraintSolver: sweep back and forward to reduce asymmetry
add missing 1.sdf for Bullet/data/kitchens/1.sdf
add support for getting keyboard events (pybullet.getKeyboardEvents and b3RobotSimulatorClientAPI::getKeyboardEvents)
