Fix some bugs in recently raised issues (capsule support, default torch device bug, and mujoco dependency)

[StoneT2000]

• Add support for capsule shapes when loading URDFs Exception: Invalid Geometric Tag: capsule #40
• Fix bug with default torch device Device error when using torch.set_default_device("cuda") #39
• Remove mujoco as a dependency so it is not installed along with everything else Is it possible to install the package without mujoco dependency? #35

[LemonPi]

Hi, thank you for your contribution! I just defended my thesis and so should have more time to maintain the code. I agree that Mujoco shouldn't be a hard dependency. However the mjcf parts should work if Mujoco is installed (see the package failing messages above). I think the best way is to add a line in .github/workflows/python-package.yml to pip install mujoco.

(I'll make the change if you don't respond within a week)