'assign' functions for Functions should not use shared pointers

[Prof Garth Wells]

For some reason, FunctionAssigner methods and the free-functions assign require shared pointer arguments even when they don't hold a pointer to the object being passed. The interface design is misleading.

If a reference is not hold, a plain reference interface should be used.

[Jan Blechta]

Didn't check the details now but I think that shared pointer is used because of Hierarchical. See #642, #643, and also #803; also note that C++17 feature std::enable_shared_from_this could be helpful here, see https://bitbucket.org/fenics-project/dolfin/issues/803/function-operator-is-missing#comment-34458052.

[Prof Garth Wells]

Ah. Would be nice if Hierarchical was removed.

[Jan Blechta]

I guess you're right. But different mechanism would be needed for handling adaptive chain. (Maybe it can just be made much more explicit rather than letting DOLFIN hiding refined objects into existing ones.) I think @martinal also had a vague opinion how to refactor Hierarchical.

[Martin Sandve Alnæs]

@chris_richardson prototyped a hierarchy model separated from the objects in the hierarchy.