Interface simplifications

A series of interface improvements

BREAKS THE CURRENT INTERFACE, but I think the improvements/simplifications are worth it:

• new load_budget() convenience function returns only Budget class, with ifo pre-set as class attribute.
• clean ups to gwinc() convenience function
• add Budget.run() method as simple interface to run all usual methods for common use case.

gwinc/nb.py

@@ -78,7 +78,7 @@ class BudgetItem:
     def __str__(self):
         # FIXME: provide info on internal state (load/update/calc/etc.)
         return '<{} {}>'.format(
-            ' '.join([c.__name__ for c in self.__class__.__bases__]),
+            ', '.join([c.__name__ for c in self.__class__.__bases__]),
             self.name,
         )