Improve freeze/thaw usability (#123)

* Clone _setup_actx to permit catching leaks

* Point arraycontext at freeze-thaw-usability in req.txt

* Point requirements.txt back to main for arraycontext

grudge/discretization.py

@@ -111,7 +111,7 @@ class DiscretizationCollection:
                  DeprecationWarning, stacklevel=2)
             discr_tag_to_group_factory = quad_tag_to_group_factory
 
-        self._setup_actx = array_context
+        self._setup_actx = array_context.clone()
 
         from meshmode.discretization.poly_element import \
                 PolynomialWarpAndBlendGroupFactory