[Dune] Description of DGF in the grid-howto

[Christian Engwer]

Dear all,

this week the grid-howto got updated and does describe the DGF
parser. I would like to comment on some of these changes.

* The howto states that it is necessary to use obscure preprocessor
  macros to use the DGF parser. I think the user should not be
  encouraged to use preprocessor macros. We want to teach then a clean
  programming stile. The preprocessor does obfuscate the code and it
  get's more difficult to debug. The examples should be adjusted such
  that the GridType macro is not used.

* The howto does only offer a very brief introduction to the DGF
  parser and refers to the doxygen documentation for all further
  information. I think a howto should explain a little more than
  this.

* The tutorial was structured in a step by step fashion. The unitcube
  class was introduced to ease the creation of different grids. Now
  some applications use the DGF parse to create these unitcubes. I
  don't really see an advantage in this behaviour. I would prefer to
  have an additional application that teachs the usage of the DGF
  parser.

Cheers Christian