Fixed problem running plotting script

If a plotting script included function (or class) definitions, it failed
due to the default behaviour of the exec() command. Now it sets globals
and locals in such a way that function defns are allowed.

src/python/pimlico/datatypes/plotting.py

@@ -5,9 +5,10 @@
 import os
 import warnings
 
-from pimlico.datatypes import NamedFileCollection
 from past.builtins import execfile
 
+from pimlico.datatypes import NamedFileCollection
+
 
 class PlotOutput(NamedFileCollection):
     """
@@ -31,7 +32,7 @@ def plot(self):
             cwd = os.getcwd()
             os.chdir(self.data_dir)
             # Execute the python script
-            execfile(self.get_absolute_path("plot.py"))
+            execfile(self.get_absolute_path("plot.py"), locals(), locals())
             # Change working dir back
             os.chdir(cwd)
             # Check that plot.pdf got created