direct_api: Joint.connected_to is now a set(), not singleton

[ezrec]

• Make Joint.connected_to a set(), instead of a singleton

This permits multiple attachments to a single joint; for example a common 'origin' virtual mounting point; or multiple attachments to a slot in 2020 aluminum extrusion.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8209624) 95.72% compared to head (f336813) 95.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #138   +/-   ##
=======================================
  Coverage   95.72%   95.72%           
=======================================
  Files          21       21           
  Lines        6455     6455           
=======================================
  Hits         6179     6179           
  Misses        276      276           

Files Changed	Coverage Δ
src/build123d/topology.py	95.52% <100.00%> (ø)

☔ View full report in Codecov by Sentry.

📢 Have feedback on the report? Share it here.

[gumyr]

Although I can see why this would be useful for linear joints like those on extrusions, it doesn't make sense to me generally. A ball and socket joint can't have two balls in the socket. Rigid joints would be confusing as two parts are likely to overlap.

Given that any part can have as many joints as desired, this seems like redundant functionality that just makes everything more difficult to understand. Is there a compelling reason by this approach is required?

[ezrec]

A ball and socket can't have two balls, but a ball can connect to two sockets (double-ball joint in claymation armatures) https://www.animationtoolkit.co.uk/12mm-double-ball-joint/

And with respect to Linear Rails, I'm working on a hardware library for my workflow, and it makes more sense to me to have a single linear joint for each side that I can attach multiple to, instead of having to instance with each specific rail with a unique joint set, ie something like:

class VSlot2020(Compound):
  def __init__(self, north_slot_joint_names = set(), south_slot_joint_name=set(), ....)
  ...
  
rail = VSlot2020(east_slot_joint_names = { "endstop", "carriage", "idler_block" })