Adding a spline joint to the joint collection #1025

Job	Run time
nix (macos)	8m 30s
nix (ubuntu, pkgs-full.py-pinocchio)	6m 55s
nix (macos, pkgs-full.py-pinocchio)	7m 0s
nix (ubuntu)	9m 13s
check-macos-linux-nix	5s
	31m 43s

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