Atl: Only import Grf if needed

Importing Grf pulls in matplotlib, which can cause conflicts, for instance
between GTK2 and GTK3 symbols.

Author: dsberry

starlink/Atl.py

@@ -1,5 +1,4 @@
 import starlink.Ast as Ast
-import starlink.Grf as Grf
 from distutils.version import LooseVersion
 
 try:
@@ -303,6 +302,8 @@ def plotframeset(axes, gbox, bbox, frameset, options=""):
           >>>    matplotlib.pyplot.show()
     """
 
+    import starlink.Grf as Grf
+
     axes.xaxis.set_visible(False)
     axes.yaxis.set_visible(False)