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Added external PCF-33P-1 thread based on proposal by dholth in #24 but
- changed order of points in profile
- flipped so that load-side of thread is on top
- changed pitch from 6.35 mm to 3.18 mm
Added internal PCF-33P-1 thread to sensibly match external thread
(spec defines only external thread, not internal thread).
Added unit tests checking slopes on load and non-load sides.
