update minitaur.py to use minitaur.urdf (instead of quadruped.urdf), also sort the legs in the same order as real hardware

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

examples/pybullet/minitaur_test.py

@@ -1,3 +1,7 @@
+import sys
+#some python interpreters need '.' added
+sys.path.append(".")
+
 import pybullet as p
 from minitaur import Minitaur
 from minitaur_evaluate import *