modernise packaging, and better manage optional depencencies

pyproject.toml

@@ -47,8 +47,9 @@ dependencies = [
 qml = ["ase"]
 regression = ["scikit-learn"]
 wigner = ["sympy"]
+gmol = ["cell2mol"]
 equio = ["equistore-core @ git+https://github.com/lab-cosmo/equistore.git@e5b9dc365369ba2584ea01e9d6a4d648008aaab8#subdirectory=python/equistore-core"]
-all = ["qstack[qml,regression,wigner,equio]"]
+all = ["qstack[qml,regression,wigner,equio,gmol]"]
 
 #dev = ["tqdm"]  # those are deps for the tests
 

qstack/compound.py

@@ -108,8 +108,18 @@ def gmol_to_mol(fin, basis="def2-svp"):
         pyscf Mole: pyscf Mole object.
     """
 
-    from cell2mol.tmcharge_common import Cell, atom, molecule, ligand, metal
-    from cell2mol.tmcharge_common import labels2formula
+    try:
+        from cell2mol.tmcharge_common import Cell, atom, molecule, ligand, metal
+        from cell2mol.tmcharge_common import labels2formula
+    except ImportError:
+            print("""
+
+ERROR: cannot import ĉell2mol. Have you installed qstack with the \gmol\" option?\n\n
+(for instance, with `pip install qstack[gmol] or `pip install qstack[all]``)
+
+""")
+    raise
+
 
     # Open and read the file
     if fin.endswith(".gmol"):