Add logic to decide whether one or two darts is necessary

[scantopattern]

Specifically on the Gill trousers, there are thresholds where the dart opening exceeds 4cm, then we really should have two darts instead of just one so we would need to add this switch in the code.

[aharwood2]

This is not a priority as it was for an older version of the Gill trousers. Will leave it for now as it might still be useful in the future.