title: Convert multidimesnional matrix to a one dimensional vector
body: <p>I have an MxN numpy matrix and would like to make it into a one dimensional vector that just stacks each row after another. Essentially something like this...</p>
  
  <pre><code>[x11, x12 ... x1m
   x21, x22 ... x2m
   .
   .
   xn1, xn2 ... xnm]
  </code></pre>
  
  <p>Goes to this...</p>
  
  <pre><code>[x11, x12 ... x1m, x21, x22 ... x2m ... xn1, xn2 ... xnm]
  </code></pre>
  
  <p>What is the best/ most efficient way to do this? </p>