Return to single thread, add default values and fix imports

There is a bottleneck at function load_model_from_xml from mujoco-py
when using multi threading. In a single thread, a call to this function
takes units of milliseconds, while in multi threading it takes tens of
milliseconds. Maybe this is due to internal data structures that are
required for both loading the model in the worker thread and performing
the simulations in the main thread, causing the delay in
load_model_from_xml and other functions that can be perceived in the
cumulative time obtained by the profiler by running
test_dynamics_rand.py.
Due to this poor performance, the file mujoco_model_gen.py was removed
since it serves no purpose now that the variations.py file contains
methods to process the XML file, and the calls to obtain the randomized
model are done directly in the class RandomizedEnv.

rllab/envs/mujoco/dynamics_randomization/randomized_env.py

@@ -1,10 +1,10 @@
 import os.path as osp
 
 from mujoco_py import MjSim
+from mujoco_py import load_model_from_xml
 
 from rllab.core import Serializable
 from rllab.envs import Env
-from rllab.envs.mujoco.dynamics_randomization import MujocoModelGenerator
 from rllab.envs.mujoco.mujoco_env import MODEL_DIR
 
 
@@ -25,16 +25,16 @@ def __init__(self, mujoco_env, variations):
         self._wrapped_env = mujoco_env
         self._variations = variations
         self._file_path = osp.join(MODEL_DIR, mujoco_env.FILE)
-        self._model_generator = MujocoModelGenerator(self._file_path,
-                                                     variations)
+        self._variations.initialize_variations(self._file_path)
 
     def reset(self):
         """
         The new model with randomized parameters is requested and the
         corresponding parameters in the MuJoCo environment class are
         set.
         """
-        self._wrapped_env.model = self._model_generator.get_model()
+        self._wrapped_env.model = load_model_from_xml(
+                self._variations.get_randomized_xml_model())
         if hasattr(self._wrapped_env, 'action_space'):
             del self._wrapped_env.__dict__['action_space']
         self._wrapped_env.sim = MjSim(self._wrapped_env.model)
@@ -65,7 +65,6 @@ def terminate(self):
         Besides regular termination, the MuJoCo model generator is
         stopped.
         """
-        self._model_generator.stop()
         self._wrapped_env.terminate()
 
     @property