Customisable Modular Model Rocket - Remixed

This is a remix/update of the amazing Customisable Modular Model Rocket thing by Damian Axford. The original hasn't been updated since 2015.

Updates:
* 06/2020: Add option to print Lego-compatible studs on the payload cavity floor :)
* 06/2020: Invert coupling between body and payload coupling and add parameters so that multiple (small enough to be printed) body sections can be stacked.
* 06/2020: Add parameter for whether shock cord attachment should be added at the top of the fin can, and bind it to the coupling height rather than the top of the motor casing height.
* 06/2020: Ensure inner_diameter isn't less than the motor diameter. This seems to have been an ongoing source of confusion.
* 06/2020: Add payload_body_bulkhead/ payload_body_shockcord_fixing parameters for the case that you wish to have a parachute in the payload coupling and wish to eject the entire payload body.
* 06/2020: Assorted tweaks along the way. Hopefully more steps forward than backward..