title: Joining ndarrays in Python where indices are floats and only approximately equal
body: <p>Let's say I have two ndarrays:</p>
  
  <pre><code>a = [[1.1,10]
       [2.2,20]
       [3.3,30]
       [4.4,40]
       [5.5,50]]
  
  b = [[1.5,100]
       [1.9,200]
       [2.3,250]
       [3.9,300]]
  </code></pre>
  
  <p>I want to join these two tables together so that I return:</p>
  
  <pre><code>c = [[1.1,10,None]
       [2.2,20,200]  #note this is 200 not 250
       [3.3,30,250]
       [4.4,40,300]
       [5.5,50,300]]
  </code></pre>
  
  <p>In other words, I want to do something like "A left join B where if the keys do not match exactly, use the key which is less than and nearest from B".</p>
  
  <p>Please forgive me if I have not formatted this question correctly, it's my first question on StackOverflow, and I'm not a professional coder.  I have searched thoroughly through StackOverflow and Google though.</p>
  
  <p>I have some sort of inkling that the answer could be somewhere between <a href="https://stackoverflow.com/questions/2774949/merging-indexed-array-in-python">merging indexed array in Python</a>
  and using a masked array.  Basically, I don't know, and please help!</p>