title: xlwings.book hangs if file is already open
body: <p>How to close a file if it is already open? </p>
  
  <pre><code>import xlwings as xw
  wb = xw.Book(folderpath + 'Metrics - auto.xlsx')
  </code></pre>
  
  <p>Using try:except: but need a way to close the file so it can be opened, or find the file and work with it? </p>
  
  <p>I get this error if it's already open:</p>
  
  <pre><code>wb = xw.Book(folderpath + 'Metrics - auto.xlsx')
  Traceback (most recent call last):
  
    File "&lt;ipython-input-34-85b6fd35627b&gt;", line 1, in &lt;module&gt;
      wb = xw.Book(folderpath + 'Metrics  - auto.xlsx')
  
    File "C:\Users\ReDimLearning\AppData\Local\Continuum\anaconda2\lib\site-packages\xlwings\main.py", line 480, in __init__
      impl = app.books.open(fullname).impl
  
    File "C:\Users\ReDimLearning\AppData\Local\Continuum\anaconda2\lib\site-packages\xlwings\main.py", line 2751, in open
      "Cannot open two workbooks named '%s', even if they are saved in different locations." % name
  
  ValueError: Cannot open two workbooks named 'metrics - auto.xlsx', even if they are saved in different locations.
  </code></pre>