Implement Non null functions

[prestoncabe]

See: https://cloud.trisotech.com/help/feel-functions/nn.html

• [ ] all(list)
• [ ] any(list)
• [ ] count(list)
• [ ] max(list)
• [ ] mean(list)
• [ ] median(list)
• [ ] min(list)
• [ ] mode(list)
• [ ] stddev(list)
• [ ] sum(list)

[prestoncabe]

@kbon1083 You could start with one of these functions? sum, count, min, or max might be easiest?

These are similar to built-in DMN functions, but they account for nulls.

[prestoncabe]

turns out that the Kogito/Drools FEEL implementation already supports "non null" versions of these functions.