Add trouser outline to Gill 3 trouser

[scantopattern]

Create a trouser pattern outline as an extension of the Gill 3 trouser draft so you get both the flat body mapping and the trouser outline

[aharwood2]

As the outline has been done in Seamly, we can simply extract the logic from Seamly to allow us to create the proper pattern outline as the points and curves are driven from the original outline we have.