set matplotlib cache directory

This MR resolves issues with matplotlib lock files.

Matplotlib uses a cache directory in the user's home directory by default. For a plotting job running in condor, the user's home directory is accessed over the network, which introduces latency and causes lock file errors. The change in this MR uses the previously empty __init__.py script in gstlal.plots to set a special matplotlib environment variable, telling it to use local tmp space for its cache directory.

gstlal/python/plots/__init__.py

-# Copyright (C) 2013-2016 Kipp Cannon
-# Copyright (C) 2015      Chad Hanna
-# Copyright (C) 2023      Cort Posnansky
+# Copyright (C) 2023 Cort Posnansky
 #
 # This program is free software; you can redistribute it and/or modify it
 # under the terms of the GNU General Public License as published by the
@@ -20,9 +18,12 @@
 import os
 
 
-# set matplotlib cache location to local tmp directory
-if os.getenv("_CONDOR_SCRATCH_DIR"):
-        os.environ['MPLCONFIGDIR'] = os.getenv("_CONDOR_SCRATCH_DIR")
-else:
-        os.environ['MPLCONFIGDIR'] = os.getenv("TMPDIR")
+def set_matplotlib_cache_directory():
+	"""
+	Change the matplotlib cache directory to a local temporary location.
+	"""
+	if os.getenv("_CONDOR_SCRATCH_DIR"):
+		os.environ['MPLCONFIGDIR'] = os.getenv("_CONDOR_SCRATCH_DIR")
+	else:
+		os.environ['MPLCONFIGDIR'] = os.getenv("TMPDIR")