add support for the new DUNE-ALUGrid module

this is basically the dune-grid grid manager for ALUGrid properly
out-sourced into an own module. (it does not require an external
library anymore, so it simplifies the build quite a bit.) Also, the
DUNE-ALUGrid module has quite a few new features and performance
improvements compared to the old code from dune-grid. For details, see

http://users.dune-project.org/projects/dune-alugrid
ewoms: make it possible to use the dune-alugrid module instead of the ALUGrid manager in dune-grid
dune-alugrid is optional, not required!