utils.stretchToScale() function calculates the scale for a given amount of
float utils.stretchToScale(float stretch)
The way people measure stretch intuitively is different from the way we handle stretch in code.
When people say fabric has 25% stretch they mean that 10 cm of fabric can stretch to 12.5 cm. In code that means we would need to scale non-stretch lengths by 80% to get the correct lengths to use in patterns with 25% stretch fabric.
Pattern designers need a way to calculate the scaling factor to use for their pattern part lengths, given a fabric stretch percentage. This function does that by returning:
1 / (1 + parseFloat(stretch))