* nut_sides must be last argument
* default value = 0. With 120 as default, we cannot set nut_sides to 120
while simultaneously setting fn to something different from 120.
Addressing issue #46: "fn needs to be passed in call to straight_thread()".
I just added the missing "fn=fn," line 36. That should take care of the issue.
I tested locally and it seems to be fine, now specifying fn in bolt(), nut() and tap() work as expected.
I ran across the mental block of adding interior threads to a pre-cut hollow, which broke my brain a bit.
I find it more natural to think of adding a threaded hole to a solid as a 'tap' or difference() operation, rather than differencing a hollow and then adding interior threads to it.
For this reason I've created the, `tap(...)` module, which creates a positive of the internal threads suitable for differencing to produce a threaded tap hole.
