Add pebble as optional dependency for parallelization

Co-authored-by: RMeli <11348981+RMeli@users.noreply.github.com>

Author: Copilot

pyproject.toml

@@ -29,6 +29,7 @@ Homepage = "https://spyrmsd.readthedocs.io"
 bib = ["duecredit"]
 rdkit = ["rdkit"]
 networkx = ["networkx"]
+parallel = ["pebble"]
 test = [
     "pytest",
     "pytest-cov",