v0.1.0-rc12 Improved Goal Tracking + Table Cleaning Tasks w/ Fixed object ρ₀

1. now sample goal offsets for tracking targets once per episode (upon reset, saved in goal_offsets cache)
2. add the following sub-tasks in the table cleaning taskset for faster experiments

# Fixing the objects in-place
register(id='Clean-i-fixed-v0', entry_point=vec_clean_env, kwargs=dict(
    initial_qpos={'object1:joint': [1.35, 0.75, .45, 0, 0., 0., 0.]},
    obj_reset={'object0': dict(pos=[1.35, 0.7, 0.42478468]),
               'object1': dict(pos=[1.35, 0.8, 0.42478468]), },
    goal_sampling={'object0': dict(target="box", range=0, ),
                   'object1': dict(target="object1", range=0, offset=[0, 0, 0]), },
), **kw)

register(id='Clean-ii-fixed-v0', entry_point=vec_clean_env, kwargs=dict(
    obj_reset={'object0': dict(track='box', avoid=['gripper'], range=0, h=0.43456914),
               'object1': dict(pos=[1.35, 0.8, 0.42478468]), },
    goal_sampling={'object0': dict(target="box", range=0),
                   'object1': dict(target="object0", range=0), },
), **kw)

register(id='Clean-train-fixed-v0', entry_point=SampleEnv,
         kwargs={'fetch:Clean-i-fixed-v0': 0.5, 'fetch:Clean-ii-fixed-v0': 0.5, }, **kw)

register(id='Clean-train-fixed-80-20-v0', entry_point=SampleEnv,
         kwargs={'fetch:Clean-i-fixed-v0': 0.8, 'fetch:Clean-ii-fixed-v0': 0.2, }, **kw)

register(id='Clean-fixed-v0', entry_point=vec_clean_env, kwargs=dict(
    obj_reset={'object0': dict(pos=[1.35, 0.7, 0.42478468]),
               'object1': dict(pos=[1.35, 0.8, 0.42478468]), },
    goal_sampling={'object0': dict(target="box", range=0),
                   'object1': dict(target="box", range=0), },
), **kw)