def import filename, extension=nil, options={}
      extension = file_extension(filename) unless extension
      if ['txt','csv','xml'].include?(extension)
        open_text filename, extension, options
      else
        open_binary filename, extension, options
      end
    end
def import filename, extension=nil, options={}
      extension = file_extension(filename) unless extension
      if ['txt','csv','xml'].include?(extension)
        open_text filename, extension, options
      else
        open_binary filename, extension, options
      end
    end
Loads an external file into an existing worbook

 @param [String] filename   a string with a reference to the file to be opened
 @param [String] extension  an optional string enforcing a certain parser (based on the file extension, e.g. 'txt', 'csv' or 'xls')
 @return [Workbook::Book] A new instance, based on the filename