Until now, the method GridFactory::createGrid returned the grid object
it created as a plain C pointer.  That way, it was not obvious from
the API who was to keep the responsibility for eventually deleting
the object: the GridFactory or the calling code.  At the 2017 Dune
Dev Meeting in Heidelberg it was decided that the method should
really return a std::unique_ptr instead, to make it clear that the
calling code gets the ownership of the new object.  To ease the
transition towards the new interface, the class ToUniquePtr was
introduced in dune-common.  Objects of that type are std::unique_ptr
objects, but they also cast to C pointers.  That way, code that calls
createGrid can still store the return value in a C pointer (but will
trigger a deprecation warning).

However, there does not seem to be a way to ensure similar compatibility
for implementors of third-party grid implementations.  Those will have
to adapt their implementations of GridFactory::createGri...