Big big refactor

[wouterwln]

Step one in making GPPL more modular such that we can use BipartiteFactorGraphs under the hood and make everything quick quick quick

Also uses TestItemRunner instead of ReTestItems to make testing more robust.
Splits up graph_engine.jl in multiple files, does the same for the tests

[codecov]

Codecov Report

Attention: Patch coverage is 89.28164% with 94 lines in your changes missing coverage. Please review.

Project coverage is 90.75%. Comparing base (a4d21be) to head (4209618).

Files with missing lines	Patch %	Lines
src/model/variable_ref.jl	82.46%	27 Missing ⚠️
src/model/context.jl	75.55%	22 Missing ⚠️
src/model/proxy_label.jl	84.61%	10 Missing ⚠️
src/nodes/node_materialization.jl	95.03%	8 Missing ⚠️
src/graph/interfaces.jl	36.36%	7 Missing ⚠️
src/model/model.jl	93.02%	6 Missing ⚠️
src/graph/node_data.jl	90.00%	3 Missing ⚠️
src/core/functional_indices.jl	92.30%	2 Missing ⚠️
src/core/node_types.jl	66.66%	2 Missing ⚠️
src/graph/node_labels.jl	93.33%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   90.64%   90.75%   +0.10%     
==========================================
  Files          16       33      +17     
  Lines        2170     2173       +3     
==========================================
+ Hits         1967     1972       +5     
+ Misses        203      201       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.