Increase test coverage (#99)

* increase torch_sim/transforms.py test coverage from 14% to 94%

* tweak doc strings

* increase torch_sim/models/soft_sphere.py coverage from 42% to 60%

* clean up and parametrize soft_sphere unit tests

* format

Author: janosh

README.md

@@ -4,7 +4,7 @@
 [![codecov](https://codecov.io/gh/radical-ai/torch-sim/branch/main/graph/badge.svg)](https://codecov.io/gh/radical-ai/torch-sim)
 [![This project supports Python 3.11+](https://img.shields.io/badge/Python-3.11+-blue.svg?logo=python&logoColor=white)](https://python.org/downloads)
 [![PyPI](https://img.shields.io/pypi/v/torch-sim?logo=pypi&logoColor=white)](https://pypi.org/project/torch-sim)
-[![Zenodo](https://img.shields.io/badge/DOI-TODO-blue?logo=Zenodo&logoColor=white)][zenodo]
+[![Zenodo](https://img.shields.io/badge/Zenodo-15127004-blue?logo=Zenodo&logoColor=white)][zenodo]
 
 [zenodo]: https://zenodo.org/records/15127004