Rigol DS1052E replacement knob small

The trigger button on my Rigol DS1052E oscilloscope broke (it cracked). So I needed a replacement one.
Changes to blackboots' model:
- made the number of ridges around the knob adjustable
- the flat side on the shaft doesn't go all the way down (it stops a couple of mm above the bottom), so I made that an adjustable parameter
Update 2017-09-05: removed some dead code.