Keep operations in grudge.op and clean headers

grudge/interpolation.py

 """
+.. currentmodule:: grudge.op
+
 Interpolation
 -------------
 

grudge/projection.py

 """
+.. currentmodule:: grudge.op
+
 Projections
 -----------
 

grudge/reductions.py

 """
-Reduction operations
-^^^^^^^^^^^^^^^^^^^^
+.. currentmodule:: grudge.op
 
 Nodal reductions
 ----------------

grudge/trace_pair.py

+"""
+Trace Pairs
+^^^^^^^^^^^
+
+Container class
+---------------
+
+.. autoclass:: TracePair
+
+Boundary trace functions
+------------------------
+
+.. autofunction:: bdry_trace_pair
+.. autofunction:: bv_trace_pair
+
+Interior and cross-rank trace functions
+---------------------------------------
+
+.. autofunction:: interior_trace_pairs
+.. autofunction:: cross_rank_trace_pairs
+"""
+
 __copyright__ = """
 Copyright (C) 2021 University of Illinois Board of Trustees
 """
@@ -45,29 +67,6 @@ import numpy as np
 import grudge.dof_desc as dof_desc
 
 
-__doc__ = """
-Trace Pairs
-^^^^^^^^^^^
-
-Container class
----------------
-
-.. autoclass:: TracePair
-
-Boundary trace functions
-------------------------
-
-.. autofunction:: bdry_trace_pair
-.. autofunction:: bv_trace_pair
-
-Interior and cross-rank trace functions
----------------------------------------
-
-.. autofunction:: interior_trace_pairs
-.. autofunction:: cross_rank_trace_pairs
-"""
-
-
 # {{{ Trace pair container class
 
 @with_container_arithmetic(