[RTM?] Added more examples of using holonomic constraints

[p-shg]

Specifically using more objects, or more than one constraint, on different objects or on the same object pair. This should help newer users of holonomic constraints understand this better

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Changes to Core Features:

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Examples include an extention of the already provided pendulum to a 2D 3 bar mechanism and a 2D 4 bar mechanism, as well as a 3D double pendulum and 2 examples of using several constraints on the same 2 solids

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This change is 

[Ipuch]

Add tests in test_all_exemples.py

[pariterre]

Please merge with master and make sure the tests pass

@pariterre reviewed 17 of 17 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @p-shg)

[pariterre]

@pariterre reviewed 14 of 14 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @p-shg)

[p-shg]

Tests should be ok on my side
RTM ?

[Ipuch]

@Ipuch reviewed 6 files and all commit messages.
Reviewable status: 19 of 20 files reviewed, all discussions resolved (waiting on @pariterre).