[gridcreator] Deprecate simplex, cube, dgf grid creator

Structured grids can be defined (since dumux 2.8 / dune 2.4)
via the input file like follows:

[Grid]
LowerLeft = 0 0 0
UpperRight = 4 5 6
Cells = 2 2 6
CellType = Simplex
Refinement = 1         # optional

Note that Simplex is only available for grids supporting simplices.
For multidomain applications the property GridParameterGroup can be
set in each sub problem if you need to define different grids for
the subdomains in the same input file.

DGF
The new default gridcreator (since dumux 2.8) has the
identical functionality with identical interface. Just specify

[Grid]
File = bla.dgf

dumux/io/cubegridcreator.hh

@@ -25,6 +25,8 @@
 #ifndef DUMUX_CUBE_GRID_CREATOR_HH
 #define DUMUX_CUBE_GRID_CREATOR_HH
 
+#warning This file is deprecated and will be removed after Dumux 2.9. Use the default (Dumux::GridCreator). Structured grids can be defined via the input file now (see handbook). Set CellType = Cube for cube grid.
+
 #include <dune/grid/utility/structuredgridfactory.hh>
 
 #include <dumux/common/basicproperties.hh>

dumux/io/dgfgridcreator.hh

@@ -23,6 +23,8 @@
 #ifndef DUMUX_DGF_GRID_CREATOR_HH
 #define DUMUX_DGF_GRID_CREATOR_HH
 
+#warning This file is deprecated and will be removed after Dumux 2.9. Use the default grid creator (Dumux::GridCreator).
+
 #include <dune/common/parallel/mpihelper.hh>
 #include <dune/grid/io/file/dgfparser.hh>
 

dumux/io/simplexgridcreator.hh

@@ -25,6 +25,8 @@
 #ifndef DUMUX_SIMPLEX_GRID_CREATOR_HH
 #define DUMUX_SIMPLEX_GRID_CREATOR_HH
 
+#warning This file is deprecated and will be removed after Dumux 2.9. Use the default (Dumux::GridCreator). Structured grids can be defined via the input file now (see handbook). Set CellType = Simplex for simplex grid.
+
 #include <dumux/common/propertysystem.hh>
 #include <dumux/common/parameters.hh>