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 endandsqrt(x). Available variables:$computed(computed grade part),$checklist(checklist grade),$combined(WebLab combined grade) and$add1,$add2, etc for additional grades