Add conftest file with fixtures for example systems.

tests/conftest.py

@@ -0,0 +1,18 @@
+import pytest
+import sire as sr
+
+
+@pytest.fixture(scope="session")
+def ethane_methanol():
+    mols = sr.load(sr.expand(sr.tutorial_url, "merged_molecule.s3"))
+    for mol in mols.molecules("molecule property is_perturbable"):
+        mols.update(mol.perturbation().link_to_reference().commit())
+    return mols
+
+
+@pytest.fixture(scope="session")
+def ethane_methanol_hmr():
+    mols = sr.load(sr.expand(sr.tutorial_url, "merged_molecule_hmr.s3"))
+    for mol in mols.molecules("molecule property is_perturbable"):
+        mols.update(mol.perturbation().link_to_reference().commit())
+    return mols