[Dune] [#609] Semantics of IteratorType Ghost_Partition

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FS#609 - Semantics of IteratorType Ghost_Partition
User who did this - Robert Klöfkorn (robertk)

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First of all, there is alsways only one layer of ghost cells. 
The method ghostSize on the grid class returns (at least in ALUGrid) the number of ghost cells present in the grid. Otherwise the method should be changed to bool ghostLayer () returing true if ghost cells are supported false otherwise (ALUGrid would return true, YaspGrid would return false).

Ghost cells are helper cells that cannot provide full entity information. 

Therefore, from my point of view GhostEntities may not have: 

1) Hierarchic iterators (although implemented for debugging reasons in ALUGrid). Since GhostEntities are not normal elements it might be not so easy to implement father-children relations for some grids. Also, even if hierarchy information is available it cannot be used for prolongation and restriction during adaptation (since on one process for example only 4 ghost children are available instead of 8, cf. ALUGrid). Data on ghost entities has to be restored with a communication procedure (usually Interior_All_Interface) after grid changes. 
Along with this the method father is not available and also the method geometryInFather is not there. 
However, methods like subEntity and Entity indices have to be supported to attach data also to ghost entities.  

2) Full IntersectionIterator not available. IntersectionIterators started on GhostEntities only return the interior element. GhostEntities do not have to provide further neighbor information. 

Did I forget something? 
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