Undercut trapezoidal threads when using external=False

[Dmytry]

Looks like this:

Note that the threads are undercut i.e. the angle is -29 degrees rather than 29 degrees.

Also, it seems that the root_radius and apex_radius are swapped (which may be why).

external=True works as expected.

[Dmytry]

On second thought that may be me not understanding how the external threads are meant to be used - in my code I was using them to cut a hole rather than adding them after cutting a hole.