Merge feat/torchsim-remove-metatrain: remove metatrain dependency

- Remove support for loading .ckpt files directly
- Remove 'pet-mad' shortcut (required metatrain)
- Update documentation to point users to upet for model export
- Remove metatrain from optional dependencies

Metatrain checkpoints should now be exported using upet before using
with torchsim: pip install upet && upet export model.ckpt

Addresses review comment from @Luthaf on PR #167

Author: HaoZeke