Customizable Flexible Coupling

Highly edited version of the original from Gyrobot and the version from MarcoAlici with independent shaft sizes.(see Remixed from).

Now contains many comments and named parameters.

**Features:**
- Top and bottom independent hole diameters
- Smooth exterior spirals for coupling
- Hex or square captive nuts
- Recessed bolt heads
- Optimisations for no support printing
- D shafts optional

**Notes:**
- 100% fill for structural strength
- PLA is less brittle than ABS for this structure.
- Print with no support (unless spiral degrees are very high).
- The longer and thinner the spirals are - the more flexible the coupler will be.
- So increase Spiral degrees for longer arms and Number of Spirals for thinner arms.
- Top and Bottom captive bolts are the same so there is no wobbling at high spin rates.