rewritten mesh creator to support more general creation

Automatically create dune grids by different strategies.

1. If a macro file name is given, try to determine by file extension which reader to use and read the grid from file
2. else if the parameter mesh->structured is given, try to construct a structured grid
3. else try to create the grid depending on the grid type

examples/init/heat.dat.2d

 heatMesh->macro file name:    ./macro/macro.stand.2d
 heatMesh->global refinements: 4
 heatMesh->min corner: 0 0
-heatMesh->max corner: 1 1
+heatMesh->max corner: 2 2
 heatMesh->num cells: 2 2
-heatMesh->dimension: 2 2
 
 heat->mesh:  heatMesh
 heat->names: u