issue_add_3rd_bin

[Hanziscool]

Currently we only have 2 bins (1 picking bin & 1 placing bin)

Need to add the third one on the left of the picking bin to implement ITER.

How are the bins loaded?

1. In picking.py we need to import BinsArena from robosuite.models.arena
2. In Arena: bins_arena.xml we have table_full_size (3-tuple): x, y, z of the table, table_friction (3-tuple): 3 mujoco friction, bin1_pos (3-tuple): Absolute cartesian coordinates
3. In picking.py __init__ we can initialise bin1_pos = (0.1, -0.25, 0.8), self.bin1_pos = np.array(bin1_pos)
4. In picking.py get_placement_initializer we have reference_pos = self.bin1_pos for pickObject