code: def GetCellValue (self, column, row = None, default = None):
                  
                  if row == None:
                          (row, column) = ParseCellSpec(column)
                  if row in self:
                          return self[row].GetCellValue(column, default)
                  return default
docs: get a cell, return default if that cell does not exist
                          note that column and row START AT 1 same as excel