fix: allow values smaller than lowest threshold

[romanoe]

Solves #58

[schmidni]

Your implementation was correct, according to the email you sent, I looked at the logic for the conditions in LossScale.js. While correct, they were overly complicated.
I simplified them a bit - the result is, that the condition (index === 0) in getPercentage is not needed anymore, since we will not pass it a value smaller than the smallest threshold. However I would leave it in there, since the function is "correct" this way and functions as expected.