Thread bug re neck_in_degrees and neck_out_degrees? #92

When I use screw_thread.thread() with the neck parameters I get:

• if neck_in_degrees == neck_out_degrees: things work as expected

but

• if neck_in_degrees != neck_out_degrees: odd shapes, not as expected
• if neck_in_degrees > 0 and neck_out_degrees == 0: float division by zero

(Divide-by-zero error is in screw_thread.py, line 113)