Add support for custom grading formula
A grading formula supports the following operators:
Submitted by Elmer van Chastelet on 7 February 2020 at 21:31+
,-
,*
,/
,( ... )
,^
, expressions:min(x, y)
,max(x, y)
,if x [>,>=,<,<=,==,~=] y then m else n end
andsqrt(x)
. Available variables:$computed
(computed grade part),$checklist
(checklist grade),$combined
(WebLab combined grade) and$add1
,$add2
, etc for additional grades