[Dune] unusual first line in alu3dgridfactory.hh

[Oliver Sander]

Dear Dune!
I just stumbled upon the following line in alu3dgridfactory.hh
(it is the first one).

#if defined ENABLE_ALUGRID && !defined DUNE_ALU3DGRID_FACTORY_HH

I understand what it is supposed to do.  However, I don't really like it 
because
it is non-standard and leads to surprising consequences.  In particular, my
doxygen documentation didn't include the ALUGridFactory. 

Could we please remove the 'define ENABLE_ALUGRID' and similar
occurrences elsewhere?

--
Oliver

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* Oliver Sander                ** email: sander at mi.fu-berlin.de        *
* Freie Universität Berlin     ** phone: + 49 (30) 838 75217           *
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