Centralize root_tables function

[gsheni]

Description

• There is currently 2 places we get the root parents:

• SDV/sdv/_utils.py

  Lines 363 to 366 in 447d0d0

  	def _get_root_tables(relationships):
  	parent_tables = {rel['parent_table_name'] for rel in relationships}
  	child_tables = {rel['child_table_name'] for rel in relationships}
  	return parent_tables - child_tables

• SDV/sdv/multi_table/base.py

  Lines 117 to 122 in 938bea0

  	def _get_root_parents(self):
  	"""Get the set of root parents in the graph."""
  	non_root_tables = set(self.metadata._get_parent_map().keys())
  	root_parents = set(self.metadata.tables.keys()) - non_root_tables
  	return root_parents

• Ideally, this code lives in one place, most likely as a private method on the MultiMetadata class.