Remove effectless consts.

Clang 3.5 pointed this out.
Enable use of Dune-ALUGrid.

ALUGrid became a Dune module, some include paths changed
and HAVE_DUNE_ALUGRID has to be used.

For a transition period, both macros are checked, as the
dune-alugrid module doesnt support cmake yet.
Remove testpoisson-globalfe

The test is (as the codebase it is testing) suffering from bitrot.
To make "make check" pass, this test is either to be deleted or
be ignored. As it consists mostly of a duplication of testpoisson,
I propose to modify testpoisson to cover globalfe, as soon as the
globalfe code is working again.