Switch Hierarchical::_parent from shared to weak ptr

[Jan Blechta]

This avoids Hierarchical::_parent being shared ptr with no deleter which is wrong.

This needs to be accompanied by switching many adapt functions from std::shared_ptr<Mesh> adapt(const Mesh& mesh) to std::shared_ptr<Mesh> adapt(std::shared_ptr<Mesh> mesh) etc. so that parent can be really set using shared instance.

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)

[Martin Sandve Alnæs]

Sounds about right.

See also https://bitbucket.org/fenics-project/dolfin/issues/643/get-rid-of-hierarchical-_self

Although I never liked the Hierarchical base class concept in the first place: a function is not a hierarchy of functions.

[Prof Garth Wells]

There is a very simple and comprehensive fix for this issue . . . .

[Jan Blechta]

Issue #878 was marked as a duplicate of this issue.