Defer setting plot backend as much as possible

This MR patches the gwinc.ifo command-line interface to defer setting the matplotlib backend until we know whether we will use show or savefig.

gwinc/ifo/__main__.py

 import argparse
 import numpy as np
-import matplotlib.pyplot as plt
 
 from . import IFOS, PLOT_STYLE
 from .. import load_budget
@@ -21,6 +20,11 @@ def main():
         help="save plot to file (.pdf/.png/.svg)")
     args = parser.parse_args()
 
+    if args.save:
+        from matplotlib import use
+        use('agg')
+    import matplotlib.pyplot as plt
+
     fig = plt.figure()
     ax = fig.add_subplot(1, 1, 1)