Filament width sensor (top) bowden

A very quick and dirty mod of the original (credit to jasonmrc). This reduces the internal friction while the filament is flowing through, while allowing a quick bowden tube connect/disconnect.

Adds:
4mm OD PTFE tube through middle
PC4-M6 thread mounts either side
about 1.5mm height to accomodate the extra tubing & mounts

I have printed successfully with this with TPU/flexi without any real adverse effects, normal caveat of YMMV.

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Post print, I had to ream out the centre hole slightly to get the PTFE tube to fit, but I suspect that was because my tube wasn't quite 4mm

PTFE tube was sized by fitting one PC4-M6 fitting one side, trial fitting, then trimming until I was happy with the fit when the 2nd PC4-M6 was screwed on.

Tube had to be glued in place (superglue/impact adhesive/anything that glues PTFE), then cut with a scalpel flush with the 'window' in the middle either side.