Code tidy.

edit_stack_form.php

lang/en/qtype_stack.php

locallib.php

stack/cas/casstring.class.php

stack/cas/casstring.units.class.php

stack/cas/connector.class.php

stack/cas/connector.dbcache.class.php

stack/graphlayout/displayprts.php

stack/maxima/stackmaxima.mac

stack/potentialresponsetree.class.php

stack/utils.class.php

tests/answertest_general_cas_test.php

tests/answertestfixtures.class.php

tests/docslib_test.php

tests/input_checkbox_test.php

tests/input_dropdown_test.php

tests/input_radio_test.php

tests/inputfixtures.class.php

tests/mathsoutputmathjax_test.php

tests/walkthrough_deferred_cbm_test.php