feat:Inner graphs

[rprospero]

When a node contains an inner graph, the user can press a button in the node to descend into the inner graph. There is a location bar at the top of the window that states what node the user is currently in with an up button to return back to the parent graph.

I'm submitting this with the following known issue: when you leave a graph, all of the nodes lose their positions. This is because the positions are only stored in the graph model, which only looks at the current node. It will be a larger change to start saving this information, but I thought that I should make this available before I go on leave next week.

This PR also includes a change to the Mac build on github actions. GitHub has upgraded to XCode 17, but XCode 16 is the last version supported by Conan 1. I've pinned XCode to version 16, but we need to migrate to Conan 2

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 2.

Benchmark suite	Current: 55ff064	Previous: 1992bb9	Ratio
BM_Box_MinimumImage<CubicBox>	45.08052598265693 ns/iter	15.968551941069263 ns/iter	2.82
BM_Box_MinimumVector<CubicBox>	31.68212582045349 ns/iter	10.592324251408856 ns/iter	2.99
BM_Box_MinimumImage<OrthorhombicBox>	41.97290521368653 ns/iter	15.74217935779684 ns/iter	2.67
BM_Box_MinimumVector<OrthorhombicBox>	29.515731793138464 ns/iter	10.578156169467235 ns/iter	2.79
BM_Box_MinimumImage<MonoclinicAlphaBox>	34.81036955866277 ns/iter	12.14130459963114 ns/iter	2.87
BM_Box_MinimumVector<MonoclinicAlphaBox>	23.312362130192845 ns/iter	7.796656688328149 ns/iter	2.99
BM_Box_MinimumImage<TriclinicBox>	36.053178722587404 ns/iter	12.514148739616806 ns/iter	2.88
BM_Box_MinimumVector<TriclinicBox>	23.40172218518524 ns/iter	9.160246501311764 ns/iter	2.55

This comment was automatically generated by workflow using github-action-benchmark.

CC: @disorderedmaterials/dissolve-devs

[trisyoungs]

Tried and tested - I think this mechanism will work well.

I may also take back what I said this morning about node positioning not being important - it will become a proper mess in the UI once we have a few nodes populated, so perhaps making the positioning persistent is more of a priority than I suggested! Not ultra-urgent, but probably near the top of the list when we come to getting the unit tests ported over.