[cmake] Removed galerkintest from CMakeLists.txt as it does not exist any more.
[amg] Make using the implicite build mode more failsafe.

If the overflow factor is not big enough, then the matrix will
through an exception during compress. Especially during parallel
runs this is problematic as we might only compute on a subset of
all processes.
Therefore this patchset changes the hierarchy building as follows:
- There is now an adjustable parameter for the overflow factor.
- If that is not sufficient, we will try to double it two times and
  retry building the matrix.
- If there is still an exception we stop the coarsening and propagate
  the error to all processor to let them abort more gracefully.