WRLS (Water Rocket Launch System) - EU edition

Water Rocket Launch System for standard PET bottles (as found in the EU, and probably elsewhere).

Inspired by [Superbeasti's WRLS](https://www.thingiverse.com/thing:3626084), with following major changes:
1. Different bottle adapter thread to match PCO-1881 thread
2. Different seal mechanism

Additional material:
- O-ring (16mm outer diameter, 11mm inner diameter). This is actually the same ring as used on Gardena garden hose couplers (or one of the clone brands).
- Piece of rope for the release pin.
- Block of wood (or something) and three 3x16mm screws to screw base down.