bump the required version of dune-uggrid to 2.7
Apparently, dune-grid needs dune-uggrid >= 2.7. Otherwise, I get
[ 81%] Building CXX object lib/CMakeFiles/dunegrid.dir/__/dune/grid/uggrid/uggridfactory.cc.o
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc: In instantiation of ‘Dune::ToUniquePtr<Dune::UGGrid<dim> > Dune::GridFactory<Dune::UGGrid<dim> >::createGrid() [with int dimworld = 2]’:
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:631:16: required from here
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:511:18: error: ‘ReleaseTmpMem’ was not declared in this scope
ReleaseTmpMem(grid_->multigrid_->theHeap, grid_->multigrid_->MarkKey);
~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:566:16: error: ‘ReleaseTmpMem’ was not declared in this scope
ReleaseTmpMem(grid_->multigrid_->theHeap, grid_->multigrid_->MarkKey);
~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc: In instantiation of ‘Dune::ToUniquePtr<Dune::UGGrid<dim> > Dune::GridFactory<Dune::UGGrid<dim> >::createGrid() [with int dimworld = 3]’:
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:632:16: required from here
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:511:18: error: ‘ReleaseTmpMem’ was not declared in this scope
ReleaseTmpMem(grid_->multigrid_->theHeap, grid_->multigrid_->MarkKey);
~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/temp/bernd/DUNE27/dune-grid/dune/grid/uggrid/uggridfactory.cc:566:16: error: ‘ReleaseTmpMem’ was not declared in this scope
ReleaseTmpMem(grid_->multigrid_->theHeap, grid_->multigrid_->MarkKey);
~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~