Update Logger semantics to reduce needless file creation

This change does 2 main things:

1. Fixes a bug where simlinks were created to logger files that were
   never created, if -N is passed (ie logger is disabled)
2. Delays the creation of logger directories and makes them conditional
   on logger enablement

lib/ramble/ramble/pipeline.py

@@ -66,12 +66,6 @@ def __init__(self, workspace, filters):
         self.log_path_latest = os.path.join(self.workspace.log_dir,
                                             f'{self.name}.latest.out')
 
-        fs.mkdirp(self.log_dir)
-
-        # Create simlinks to give known paths
-        self.create_simlink(self.log_dir, self.log_dir_latest)
-        self.create_simlink(self.log_path, self.log_path_latest)
-
         self._software_environments = ramble.software_environments.SoftwareEnvironments(workspace)
         self.workspace.software_environments = self._software_environments
         self._experiment_set = workspace.build_experiment_set()
@@ -127,6 +121,11 @@ def _execute(self):
 
         num_exps = self._experiment_set.num_filtered_experiments(self.filters)
 
+        if logger.enabled:
+            fs.mkdirp(self.log_dir)
+            # Also create simlink to give known paths
+            self.create_simlink(self.log_dir, self.log_dir_latest)
+
         if self.suppress_per_experiment_prints and not self.suppress_run_header:
             logger.all_msg(f'  Log files for experiments are stored in: {self.log_dir}')
 
@@ -192,6 +191,9 @@ def run(self):
             )
 
         logger.add_log(self.log_path)
+        if logger.enabled:
+            self.create_simlink(self.log_path, self.log_path_latest)
+
         self._prepare()
         self._execute()
         self._complete()

lib/ramble/ramble/util/logger.py

@@ -35,7 +35,7 @@ def __init__(self):
     def add_log(self, path):
         """Add a log to the current log stack
 
-        Opens (with 'w+' permissions) the file provided by the 'path' argument,
+        Opens (with 'a+' permissions) the file provided by the 'path' argument,
         and stores both the path, and the opened stream object in the current stack
         in the active position.