Closes #70

If DISPLAY environ not found, then default to the Agg matplotlib
backend. Useful for running on clusters etc.

tupak/__init__.py

 from __future__ import print_function, division
 
-# import local files
+# import local files, utils should be imported first
 from . import utils
 from . import detector
 from . import prior
 from . import source
 from . import likelihood
 from . import waveform_generator
-from . import sampler
\ No newline at end of file
+from . import sampler
+

tupak/utils.py

@@ -543,6 +543,14 @@ def set_up_command_line_arguments():
 
 command_line_args = set_up_command_line_arguments()
 
+if 'DISPLAY' in os.environ:
+    pass
+else:
+    logging.info('No $DISPLAY environment variable found, so importing \
+                  matplotlib.pyplot with non-interactive "Agg" backend.')
+    import matplotlib
+    matplotlib.use('Agg')
+