'Adds the given usage_key to the given block_relations map.
Arguments:
block_relations (dict({UsageKey: _BlockRelations})) -
Internal map of a block\'s usage key to its
parents/children relations.
usage_key (UsageKey) - Usage key of the block that is to
be added to the given block_relations.'

@staticmethod
def _add_block(block_relations, usage_key):

    if (usage_key not in block_relations):
        block_relations[usage_key] = _BlockRelations()
