Check that the parent is not already provided in the netcdf file

[mortenwh]

It seems we're not checking if a parent is already added from the nc-file. See:

py-mmd-tools/py_mmd_tools/nc_to_mmd.py

Lines 1534 to 1550 in 81d67c7

	self.metadata['related_dataset'] = self.get_related_dataset(
	mmd_yaml.pop('related_dataset'), ncin)
	# Add parent from function kwarg
	if parent is not None:
	if ":" not in parent:
	raise ValueError("parent must be composed as <%s>:<uuid>" %
	self.ACDD_NAMING_AUTH)
	nauth, uuid = parent.split(":")
	if nauth not in self.VALID_NAMING_AUTHORITIES:
	raise ValueError('%s ACDD attribute %s is not valid' %
	(self.ACDD_NAMING_AUTH, nauth))
	if not Nc_to_mmd.is_valid_uuid(uuid):
	raise ValueError("UUID part of the parent ID is not valid")
	self.metadata['related_dataset'].append({
	'id': parent,
	'relation_type': "parent",
	})

Two cases may appear:

1. The parent is already added - and we get repetition of the same related_dataset line. This needs to be avoided.
2. A parent is already added from the nc-file but it is not the same as the one provided in the kwargs. In this case, we should issue a warning.