Thresholds for lines of code per function

@mvankessel-EMC what do you think we should consider for thresholds for low/ medium/ high complexity for lines of code in a function? What about 
- low complexity: less than 20
- medium complexity: 20 to 99
- high complexity: 100 or more

This is entirely arbitrary, but I think some simple thresholds would be nice (in a similar way as we have for cyclomatic complexity)