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/ifo/__init__.py

 import os
 import logging
 
-from ..struct import load_struct, STRUCT_EXT
+from ..struct import Struct
 from ..util import load_module
 
 
@@ -42,9 +42,9 @@ def load_ifo(name_or_path):
         path = name_or_path.rstrip('/')
         bname, ext = os.path.splitext(os.path.basename(path))
 
-        if ext in STRUCT_EXT:
+        if ext in Struct.STRUCT_EXT:
             logging.info("loading struct {}...".format(path))
-            ifo = load_struct(path)
+            ifo = Struct.from_file(path)
             bname = 'aLIGO'
             modname = 'gwinc.ifo.aLIGO'
             logging.info("loading budget {}...".format(modname))
@@ -69,7 +69,7 @@ def load_ifo(name_or_path):
     ifo = getattr(mod, 'IFO', ifo)
     ifopath = os.path.join(modpath, 'ifo.yaml')
     if not ifo and ifopath:
-        ifo = load_struct(ifopath)
+        ifo = Struct.from_file(ifopath)
     freq = getattr(mod, 'FREQ', None)
     plot_style = getattr(mod, 'PLOT_STYLE', PLOT_STYLE)