Add assume_real as a full option in STACK.

backup/moodle2/backup_qtype_stack_plugin.class.php

db/upgrade.php

doc/en/CAS/Equivalence_reasoning.md

edit_stack_form.php

lang/en/qtype_stack.php

questiontype.php

settings.php

stack/input/equiv/equiv.class.php

stack/input/inputbase.class.php

stack/maxima/stackmaxima.mac

stack/options.class.php

tests/helper.php

tests/input_equiv_test.php

tests/questiontype_test.php

version.php