generate_exam: Generate Exam Files
In umatter/examr: Create Multiple Choice Exams

Generates pandoc documents for exam and solutions

1	generate_exam(x)

`author`	character string, the author of the exam
`title`	character string, the title of the course/exam
`date`	character string, the date of the exam (by default the system date)
`output`	character string, indicating in what format the exam and solution should be generated. either 'markdown', 'html', 'pdf', 'odt' or 'docx'. Default:pdf
`file`	character, the path/filename of the excel-sheet containing the questions to be read
`year`	character vector indicating from which years questions should be selected, if empty (NULL) questions from all years are selected.
`mock`	logical, if TRUE, only mock exam questions are selected (FALSE by default)
`topic`	character vector, indicating which question topics should be selected ('topic' column), defaults to NULL
`types`	list, indicates how many questions should be selected from each type, default: one each
`type_headers`	list, indicating the headers to be used for each question type section (see example)