use DIRECTLY_UPDATE_VELOCITY_DURING_SOLVER_ITERATIONS by default for now,
until we find the issue with some failing test cases in btMultiBody
fix a crashing issue in MyMultiBodyCreator.cpp (uninitialized variable)
disable excessive debug printf in URDF2Bullet
add importer support for FIXED_CONSTRAINT_TYPE, to be replaced by btGeneric6DofSpring2Constraint
btFixedConstraint now derives from btGeneric6DofSpring2Constraint