code: def index_of(self, data):
          
          current_node = self._first_node
          pos = 0
          while current_node:
              if current_node.data() == data:
                  return pos
              else:
                  current_node = current_node.next()
                  pos += 1
          return -1
docs: Finds the position of a node in the list. The index of the first
          occurrence of the data is returned (indexes start at 0)
  
          :param data: data of the seeked node
          :type: object
          :returns: the int index or -1 if the node is not in the list