Parent references not set for obj.metadata

[achilleas-k]

When accessing an object's metadata, the _parent reference is never set. See the Block.metadata property for example:

nixpy/nixio/block.py

Line 582 in 9b63d54

return Section(None, self._h5group.open_group("metadata"))

Objects accessed through a Container, on the other hand, always get instantiated with a parent reference:

nixpy/nixio/container.py

Line 33 in 9b63d54

return self._itemclass(self._parent, item)

This causes issues when attempting to traverse the parent chain:

nixpy/nixio/entity.py

Lines 108 to 115 in 9b63d54

	def definition(self, d):
	util.check_attr_type(d, str)
	self._h5group.set_attr("definition", d)
	par = self._parent
	while isinstance(par, Entity):
	par = par._parent
	if par.time_auto_update:
	self.force_updated_at()

We either need to make metadata Section instantiation always set the parent or have a dynamic way of creating the parent link when it's missing.

This is the source of the bug in a recent Neo PR:
NeuralEnsemble/python-neo#806