* Add test_class_sh_property_non_owning.cpp,py
Failing:
```
__________________________________________________________ test_persistent_holder __________________________________________________________
def test_persistent_holder():
h = m.DataFieldsHolder(2)
> c = h.vec_at(0).core_fld
E RuntimeError: Non-owning holder (loaded_as_shared_ptr).
h = <pybind11_tests.class_sh_property_non_owning.DataFieldsHolder object at 0x7fabab516470>
test_class_sh_property_non_owning.py:6: RuntimeError
__________________________________________________________ test_temporary_holder ___________________________________________________________
def test_temporary_holder():
d = m.DataFieldsHolder(2).vec_at(1)
> c = d.core_fld
E RuntimeError: Non-owning holder (loaded_as_shared_ptr).
d = <pybind11_tests.class_sh_property_non_owning.DataField object at 0x7fabab548770>
test_class_sh_property_non_owning.py:13: RuntimeError
```
* Introduce `shared_ptr_from_python(responsible_parent)` and use in all `property_cpp_function`s with `const shared_ptr<T> &` arguments.
Tests are incomplete.
* Complete tests.
* Add comment for `smart_holder_type_caster_load<T>::shared_ptr_from_python`
