Add set_scale to Arena and MujocoObject

[kevin-thankyou-lin]

What this does

Add ability to set_scale of bodies in arena, as well as set_scale for MujocoObjects; previously only MujocoXMLObject had these functions.

How to test

In _load_arena() add something like:

        table = mujoco_arena.worldbody.find("./body[@name='table']")
        mujoco_arena.set_scale([0.5, 0.5, 0.5], table)

scale=0.5

scale=1.5

[Abhiram824]

Any reason why you can't just move the code to a parent class? Seems like its repeated across 3 classes.