[LPD] [Dark] Don't use multiprocessing inside multiprocessing when opening run

Description

There was an error opening the run if its JSON cache file didn't already exist, because it tried to use multiprocessing inside multiprocessing, which is not allowed. This avoids that scenario. Behaviour should be unchanged if the cache file exists.

How Has This Been Tested?

No specific tests, but the parallelize=False option was added for precisely this use case - where you're already using a multiprocessing pool outside opening the run - so I'm confident in it.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@schmidtp @ahmedk

notebooks/LPD/LPDChar_Darks_NBC.ipynb

@@ -183,14 +183,18 @@
     "\n",
     "# the actual characterization\n",
     "def characterize_module(run_path, channel, gg):\n",
-    "    run = RunDirectory(run_path)\n",
+    "    run = RunDirectory(run_path, parallelize=False)\n",
     "    det_source = source_name.format(karabo_id, channel)\n",
     "    data = run[det_source, 'image.data'].drop_empty_trains()\n",
     "    data = data[skip_first_ntrains : skip_first_ntrains + ntrains]\n",
     "    cell_ids = run[det_source, 'image.cellId'].drop_empty_trains()\n",
     "    cell_ids = cell_ids[skip_first_ntrains : skip_first_ntrains + ntrains]\n",
-    "    \n",
-    "    if len(data.train_ids) < min_trains:\n",
+    "\n",
+    "    # If there is no data available, return and expect this\n",
+    "    # module to be skipped later.\n",
+    "    if len(data.train_ids) == 0:\n",
+    "        return None, None, None, None, None, None, None, None\n",
+    "    elif len(data.train_ids) < min_trains:\n",
     "        raise Exception(f\"Run {run_path} only contains {len(data.train_ids)} trains, but {min_trains} required\")\n",
     "\n",
     "    im = data.ndarray()\n",