Implement splatting variables

[wouterwln]

The implementation is slightly ugly since I'm creating a vector of ProxyLabels which I am splatting again, but I think this is the issue with splatting altogether. At least this implementation works and is supposed to fix the issue at hand. Also @bvdmitri do you have an alternative for my implementation of Base.iterate for VariableRefs? I couldn't get it to work without duplicating all code unfortunately.

[bvdmitri]

It looks fine, I thought of a different implementation to be honest, but this seems to work as well, so it shouldn't be a big problem I guess. I also added a couple of extra tests, please double check if those make sense.

P.S. Regarding the implementation I had in mind is to have Splat as an index in the proxylabel, not as a wrapper for proxied. Later on we could dispatch on the type of the index when creating the node, but your implementation seems to work fine too.

[bvdmitri]

(I guess docs build may fail because the new struct is not present in the documentation as a GraphPPL.Splat)

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.

Project coverage is 90.61%. Comparing base (4807717) to head (4fcf2c8).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/graph_engine.jl	57.89%	8 Missing ⚠️
src/model_macro.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
- Coverage   90.86%   90.61%   -0.25%     
==========================================
  Files          15       15              
  Lines        2090     2121      +31     
==========================================
+ Hits         1899     1922      +23     
- Misses        191      199       +8     

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[bvdmitri]

@wouterwln the iterate function is not actually being used (as coverage suggests), maybe we remove it?