Merge from master.

doc/content/stack.bib

doc/content/stack_publications.pdf

doc/content/stack_publications.tex

stack/cas/cassession.class.php

stack/cas/connector.class.php

stack/input/dropdown/dropdown.class.php

stack/maxima/stackmaxima.mac

tests/answertest_general_fixtures_test.php

tests/cassession_test.php

tests/castext_test.php

tests/fixtures/test_base.php

tests/helper.php

tests/input_units_test.php

tests/studentinput_test.php

tests/subscript_test.php

tests/test_base_test.php

tidyquestion.php