title: Find all submatrices of a given matrix
body: <p>I need to find all possible sub-matrices of a given matrix mxn. I am trying to do this in python and don't want to use numpy. Can we do this using loops only? </p>
  
  <p>Eg: 2x2 matrix</p>
  
  <pre><code>Matrix = [
            [1, 2], 
            [3, 4]
           ]
  
  Submatrices =[ [1], 
                 [1,2], 
                 [2], 
                 [3], 
                 [4], 
                 [3, 4], 
                 [[1], [3]], 
                 [[2],[4]], 
                 [[1,2],[3,4]] ] 
  </code></pre>