title: Scientific notation being read as string in pandas
body: <p>I'm trying to read a .csv with a column containing numbers in scientific notation.
  No matter what I do, it ends up reading them as string:</p>
  
  <pre><code>def readData(path, cols):
      types  = [str, str, str, str, np.float32]
      t_dict = {key: value for (key, value) in zip(c, types)}
  
      df = pd.read_csv(path, header=0, sep=';', encoding='latin1', usecols=cols, dtype=t_dict, chunksize=5000)
  
      return df
  
  c = [3, 6, 7, 9, 16]
  df2017_chunks = readData('Data/2017.csv', c)
  
  def preProcess(df, f):    
      df.columns = f
      df['id_client'] = df['id_client'].apply(lambda x: str(int(float(x))))
  
      return df
  
  f = ['issue_date', 'channel', 'product', 'issue', 'id_client']
  
  df = pd.DataFrame(columns=f)
  for chunk in df2017_chunks:
      aux = preProcess(chunk, f)
      df = pd.concat([df, aux])
  </code></pre>
  
  <p>How can I proper read this data?</p>