CMake search for storm-carl version.

[sjunges]

Currently, we search only in the CMake registry for carl.
If storm builds carl, then carl is exported to the registry. This has some unpleasant side-effects.

Thus besides searching the registry for carl, it would be interesting to search for storm, ask whether storm has installed carl, and get that version. Then storm no longer needs to write the carl version to the cmake registry.

As a downside, storm really is not a dependency of pycarl, so it might be a bit strange to do this. On the other hand, I feel that the combination pycarl/storm is currently rather typical.

[sjunges]

We may want to review this, as we now run on a carl-storm fork anyways..