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FS#183 - Documentation for TwistUtility missing
User who did this: - Robert Klöfkorn (robertk)

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The TwistUtility provides the twist for the inside and outside
entities of an intersection. 
Also it provides about the conformity of the intersection which cannot
be determined by only comparing level of the entities neighbouring the
intersection. These 3 features should be part of the
IntersectionIterator Interface. But due to the feature freeze, this
information has to be provided by some Utility class, i.e the
TwistUtility. 

So what is a twist again? The Twist tells, how an face, or edge is
twisted compared to the elements holding pointers to this face. 
For example in ALUGrid each face exists as an object. A Tetraeder
consits of 4 faces. A face consists of 3 edges and 1n edge consists of
2 vertices. 
So now the tetrahedrons are build following the reference tetrahedron.
But as the faces exists as real objects, vertex 0 of a face not
necessarily has to be vertex 0 of face i of the tetrahedron, it also
can be vertex 1, that would mean the twist, is 1 (or maybe -1). So
this twist tells how one face has to be twisted to fit in the
reference tetrahedron. So I hope this helps a little. 
For further reference I suggest to read the thesis of B.Schupp, page
50 ff (http://www.freidok.uni-freiburg.de/volltexte/68/). 

Again, within the dune-fem module the twist is needed for caching of
base function evaluations on quadrature points located on
faces/intersections.
As this twist is a grid feature, I think this file should be located
with the dune-grid module, although it is not needed by any other
class in the grid module. 

As you can see (by looking into the code) there is a default
implementation of this class just doing the exact thing for grids that
don't have twisted faces. For AlbertaGrid and ALUGrid, it's a little
bit different, and, as you also can see, there are a specializations
for these grids. 

How can I tell whether it is supported? The user does not have to
know, because of the default implementation. The developer on the
other hand should know what his grid does. 
Will the code not compile? No.
Do I get a run-time error? No. 
Or just silently wrong results? No. 

Any questions left? 


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