title: Create a matrix made of matrices with different sizes
body: <p>I want to create a matrix which elements are matrices (with different sizes), vectors and numbers. 
  For example, I have the next two matrices, one vector and one number:   </p>
  
  <pre><code>A = [1 2 3
       4 5 6
       7 8 9]
  
  B = [10 11
       12 13]
  
  C = [14
       15
       16]
  
  D = 17
  </code></pre>
  
  <p>And I would like to obtain a matrix 2x2, K, whith elements:<br>
   <code>k_11 = A, k_12 = B, k_21 = C, k_22 = D</code>.     </p>
  
  <p>The idea is to have the possibility to choose matrices, vectors or numbers of the big matrix, K, like they would be "simple" elements of a matrix. I.e.:
  <code>K[0,0] = A, K[0,1] = B</code> and so on.     </p>
  
  <p>Firstly, I thougth I could to obtain a list of matrices, vectors and numbers with <code>K.append(A)</code>..., but then I figured out that I will not be able to transform the list into a matrix.<br>
  And secondly, I tried to create a block matrix with <code>numpy.bmat</code>. The problem with <code>bmat</code> is that the dimensions of the inputs elements must match exactly.<br>
  Any idea?  </p>
  
  <p>Thanks. </p>