code: def getdarkcurrent(self,chip):
          
          darkcurrent = 0.
  
          try:
              darkcurrent = self._image[self.scienceExt, chip].header['MEANDARK']
  
          except:
              msg =  "#############################################\n"
              msg += "#                                           #\n"
              msg += "# Error:                                    #\n"
              msg += "#   Cannot find the value for 'MEANDARK'    #\n"
              msg += "#   in the image header.  WFC3 input images #\n"
              msg += "#   are expected to have this header        #\n"
              msg += "#   keyword.                                #\n"
              msg += "#                                           #\n"
              msg += "# Error occured in WFC3UVISInputImage class #\n"
              msg += "#                                           #\n"
              msg += "#############################################\n"
              raise ValueError(msg)
  
          return darkcurrent
docs: Return the dark current for the WFC3 UVIS detector.  This value
          will be contained within an instrument specific keyword.
  
          Returns
          -------
          darkcurrent: float
              The dark current value with **units of electrons**.