add in center_scale_burdens option from config

pipelines/cv_training/cv_training_association_testing.snakefile

@@ -18,6 +18,7 @@ burden_phenotype = phenotypes[0]
 n_burden_chunks = config.get("n_burden_chunks", 1) if not debug_flag else 2
 n_regression_chunks = config.get("n_regression_chunks", 40) if not debug_flag else 2
 n_avg_chunks = config.get('n_avg_chunks', 1)
+center_scale_burdens = '--center-scale-burdens ' if config.get('center_scale_burdens', True) else ''
 n_trials = config["hyperparameter_optimization"]["n_trials"]
 n_bags = config["training"]["n_bags"] if not debug_flag else 3
 n_repeats = config["n_repeats"]