Flexible loopsolver monitoring

This adds a flexible mechanism to supply custom termination criteria and logging output for the LoopSolver.

Notice that Max and Elias did some more work on this by also trying to replace the old hard-wired criteria with this mechanism. In principle I'd also want this, but it's much more invasive and involves some non-trivial design decisions.

In contrast, this merge request is non-invasive in the sense that it just adds the possibility to add new criteria while leaving the old behaviour untouched. If you don't use the new mechanism everything will work as before. This allows to use the feature now and incrementally improve it in the way proposed by Max and Elias later on. Since this may also require some changes, the new feature is marked as experimental.