expose "mj_setConst" functionality. (#386)

* Initialize perturbations properly.

* Expose "set constants" mujoco operation.

mujoco_py/mjrendercontext.pyx

@@ -1,6 +1,7 @@
 from threading import Lock
 from mujoco_py.generated import const
 
+
 cdef class MjRenderContext(object):
     """
     Class that encapsulates rendering functionality for a
@@ -35,6 +36,7 @@ cdef class MjRenderContext(object):
         maxgeom = 1000
         mjv_makeScene(self._model_ptr, &self._scn, maxgeom)
         mjv_defaultCamera(&self._cam)
+        mjv_defaultPerturb(&self._pert)
         mjv_defaultOption(&self._vopt)
         mjr_defaultContext(&self._con)
 
@@ -55,8 +57,11 @@ cdef class MjRenderContext(object):
         self.cam = WrapMjvCamera(&self._cam)
         self.vopt = WrapMjvOption(&self._vopt)
         self.con = WrapMjrContext(&self._con)
+
         self._pert.active = 0
         self._pert.select = 0
+        self._pert.skinselect = -1
+
         self.pert = WrapMjvPerturb(&self._pert)
 
         self._markers = []

mujoco_py/mjsim.pyx

@@ -105,6 +105,13 @@ cdef class MjSim(object):
         with wrap_mujoco_warning():
             mj_forward(self.model.ptr, self.data.ptr)
 
+    def set_constants(self):
+        """
+        Set constant fields of mjModel, corresponding to qpos0 configuration.
+        """
+        with wrap_mujoco_warning():
+            mj_setConst(self.model.ptr, self.data.ptr)
+
     def step(self, with_udd=True):
         """
         Advances the simulation by calling ``mj_step``.