forgotten __init__ move

examples/PySDM_examples/Erinin_et_al_2025/__init__.py

@@ -0,0 +1,4 @@
+# pylint: disable=invalid-name
+"""
+expansion chamber example based on [Erinin et al. 2025](https://doi.org/10.48550/arXiv.2501.01467)
+"""