title: Using bool array mask, replace False values with NaN
body: <p>I have two arrays,</p>
  
  <pre><code>a = array([
     [ 0.93825418,  0.60731973,  0.44218921,  0.90888805,  0.97695114],
     [ 0.27422807,  0.75870153,  0.12154102,  0.89137678,  0.04257262],
     [ 0.32855867,  0.17215507,  0.00302302,  0.95395069,  0.02596567],
     [ 0.18385244,  0.09108341,  0.27925367,  0.0177183 ,  0.41035188],
     [ 0.87229432,  0.73573982,  0.98554476,  0.72321398,  0.98316711],
     [ 0.16474265,  0.5308054 ,  0.27913615,  0.59107689,  0.6480463 ],
     [ 0.88356436,  0.22343885,  0.74900285,  0.43895017,  0.74993129],
     [ 0.08097611,  0.48984607,  0.33991052,  0.06431022,  0.10753135],
     [ 0.67351561,  0.13165046,  0.41327765,  0.21768539,  0.7337069 ],
     [ 0.65609999,  0.06241059,  0.3400624 ,  0.13234171,  0.23679716]
  ])
  
  b = array([
     [False,  True,  True, False, False],
     [ True, False, False, False, False],
     [ True,  True, False, False, False],
     [False, False,  True, False,  True],
     [False, False, False,  True, False],
     [False,  True,  True,  True,  True],
     [False,  True, False,  True,  True],
     [False,  True,  True, False, False],
     [ True,  True,  True,  True,  True],
     [ True, False,  True, False,  True]
  ], dtype = bool)
  </code></pre>
  
  <p>Now I want using <code>b</code> to mask <code>a</code>, retain the <code>True</code> value in <code>a</code>, and replace the <code>False</code> value with <code>NaN</code>, getting a new array that has a shape like <code>a</code>.</p>
  
  <p>How to do that?</p>