class_docstr: Undirected graph structure that stores neural gas network's
  neurons and connections between them.
  
  Attributes
  ----------
  edges_per_node : dict
      Dictionary that where key is a unique node and value is a list
      of nodes that connection to this edge.
  
  edges : dict
      Dictonary that stores age per each connection. Ech key will have
      the following format: ``(node_1, node_2)``.
  
  nodes : list
      List of all nodes in the graph (read-only attribute).
  
  n_nodes : int
      Number of nodes in the network (read-only attribute).
  
  n_edges : int
      Number of edges in the network (read-only attribute).
code: def __init__(self):
      self.edges_per_node = {}
      self.edges = {}
  
imports: from operator import attrgetter
  import numpy
  from neupy.utils import format_data
  from neupy.exceptions import StopTraining
  from neupy.algorithms.base import BaseNetwork
  from neupy.core.properties import NumberProperty, ProperFractionProperty, IntProperty
label_desc: Use image node_id 1 to create a new NeuralGasGraph object from inherited base classes: object with example: obj = NeuralGasGraph()