Add Aiken format to the possible question export formats

You need a course with some questions, including some multichoice single answer and multichoice multi answers questions. It would also be good that some multichoice single answer questions include HTML formatting and images to verify that the export is also correct for those questions. Export the questions. Verify that only multichoice single answer questions are exported and that the exported text file conforms to the Aiken syntax, see  https://docs.moodle.org/36/en/Aiken_format (Don't forget to verify that the spacing is correct, Aiken format is very picky about this). Try to import the file in another course and verify that the imported questions are OK (Keep in mind that a lot of question's elements are not exported by Aiken Format which is a very simple format, for instance all images, links, feedbacks, hints, ... are lost during the export process. This is not a bug, it's a limitation of the format. Also the name of the question is lost during the export process so a default name is derived from the question's text during import, that is not a bug, that is the expected result) Try the same process with a question bank category not containing any multichoice single answer question and verify that an error message saying that " No questions were found that could be exported. Make sure that you have selected a category to export that contains questions. " is displayed. Verify that if if there is a multichoice single answer with more than 26 answers it is not exported as the Aiken format is not able to handle it. Don't bother to create such a question, you can just import the too_many_answers.xml file attached to this issue using Moodle XML format.