[bug] don't create backup directory or display misleading log messages

when using --overwrite instead of --backup

The fact that the same method is being used both for --backup and
--overwrite is a bit messy, but there's enough overlap in their
functionality that I keep it as is for now.

dnadna/simulator.py

@@ -477,13 +477,14 @@ class Simulator(ConfigMixIn, Pluggable, metaclass=abc.ABCMeta):
         while pth.exists(backup_dir):
             backup_dir = backup_dir_base + f'.{idx}'
 
-        log.warning(
-            f'The existing scenario params file {self.scenario_params_path} '
-            f'and all existing simulation data (.npz files) will be backed '
-            f'up to {backup_dir}; you can remove this directory manually if '
-            f'you no longer require its contents')
+        if backup:
+            log.warning(
+                f'The existing scenario params file {self.scenario_params_path} '
+                f'and all existing simulation data (.npz files) will be backed '
+                f'up to {backup_dir}; you can remove this directory manually if '
+                f'you no longer require its contents')
 
-        os.makedirs(backup_dir)
+            os.makedirs(backup_dir)
 
         def backup_or_delete(src):
             if not pth.isabs(src):
@@ -504,7 +505,8 @@ class Simulator(ConfigMixIn, Pluggable, metaclass=abc.ABCMeta):
                 os.rmdir(dirname)
 
         # Back up the scenario params file to the backup directory
-        log.info('Backing up old scenario params')
+        if backup:
+            log.info('Backing up old scenario params')
         backup_or_delete(self.scenario_params_path)
 
         # Reuse the machinery from this class for iterating over files in a
@@ -516,11 +518,13 @@ class Simulator(ConfigMixIn, Pluggable, metaclass=abc.ABCMeta):
         # config file (files that do not match the format are left alone,
         # assuming they will not conflict with later attempts to read the data
         # set.  Users are responsible for cleanup of any additional junk files.
-        log.info('Backing up old scenario data')
+        if backup:
+            log.info('Backing up old scenario data')
         for _, _, filename in source._iter_dataset_files():
             backup_or_delete(filename)
 
-        log.info('Backup complete')
+        if backup:
+            log.info('Backup complete')
 
 
 class DefaultSimulator(Simulator):