Bug nchains resclust (#332)

* changed n_segments to u.chainIDs

* removed conversion to list

* clarified chain error message

src/arctic3d/cli_resclust.py

@@ -165,9 +165,14 @@ def main(input_arg, residue_list, chain, threshold, linkage, criterion):
     unique_sorted_resids = u.resids
     log.info(f"retrieved residues: {unique_sorted_resids}")
 
-    n_chains = u.n_segments
+    n_chains = len(set(u.chainIDs))
+
     if n_chains != 1:
-        log.error(f"Number of consistent segments ({n_chains}) != 1.Aborting.")
+        log.error(
+            f"Number of consistent chains ({n_chains}) != 1."
+            "Please use the --chain option to specify the chain ID."
+        )
+
         sys.exit(1)
     if len(unique_sorted_resids) != 1:
         # do the clustering