title: Find all nodes with defined attribute in networkx graph
body: <p>I looking for a elegant way to find all nodes with a defined attribute. E.g. let's say I create a new network with two nodes</p>
  
  <pre><code>G.add_node('A', attr1='alpha')
  G.add_node('B', attr1='beta')
  </code></pre>
  
  <p>Now, I would like to have a function that returns all nodes where the attribute "attr1" that matches "beta" something like</p>
  
  <p>THX
  Lazloo</p>