paper cut: define clsuter methods possibilities properly

[wwood]

+                    Arg::new("dereplication-cluster-method")
+                        .long("dereplication-cluster-method")
+                        .value_parser(["fastani", "skani"])
+                        .default_value(galah::DEFAULT_CLUSTER_METHOD),

Don't hard-code in coverm, but refer to galah's