Feature request
Put Maxima code in place to insert *s into variable names.
Fix unit tests
Change the data type of "insert stars" from a Boolean to an integer.  This is a MAJOR CHANGE affecting many parts of the code. It includes an important bug fix around line 243 of castext.class.php where two boolean variables were interchanged.  Having different datatypes here threw an exception, which is helpful.
Feature to assume all variable names typed in my the student are single characters has been added and tested, including addition of unit tests.  Importing questions does not work.
Fix import.
Code tidy.
Refactor option menus so the choices are only liarws in one place.

Also, improve the wording of the insert stars option.
Fix codechecker issues.
Fix notices when creating a new question.
Remove unneeded function argument in the test code.
Rewrite to improve the hints/fact sheets code.

We decided to rename STACK hints -> fact sheets, to avoid confusions
with Moodle interactive behaviour hints.

The display is now done using Moodle's print_collapsible_region
function.

The code that ensures that facts get displayed been moved out of
CAStext, and into maths display area.

The code that controls how the hints are displayed has been moved into
th renderer.
Merge branch 'Single_char_variable_names'
Merge branch 'wip_hints_to_factsheets'
Minor bug fix to expandp test - an odd one involving extra evaluation of arguments within functions.
Bugfix to plots
Git has failed to switch branches correctly.
Merge branch 'master' into auto_maxima_optimise