TropicalGeometry: tropical linear spaces from graphs

[YueRen]

This pull requests adds a new constructor for tropical linear spaces from graphs. Useful (for me) for example for graph rigidity questions (see e.g. here).

[YueRen]

@ollieclarke8787 Would you be able to review this pull request?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.55%. Comparing base (3e86180) to head (9079fd8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4445   +/-   ##
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  Coverage   84.55%   84.55%           
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  Files         672      672           
  Lines       88880    88886    +6     
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+ Hits        75152    75158    +6     
  Misses      13728    13728           

Files with missing lines	Coverage Δ
src/TropicalGeometry/linear_space.jl	81.87% <100.00%> (+0.76%)	⬆️

... and 2 files with indirect coverage changes

[ollieclarke8787]

Code correctly produces Bergman fan of the cycle matroid the graph. The example of K_3 is also correct.

[YueRen]

@benlorenz I've changed scalar_types_extended:

-const scalar_types_extended = Union{scalar_types,ZZRingElem}
+const scalar_types_extended = Union{scalar_types,IntegerUnion}

However, it creates a bunch of errors in the type tests that I don't know how to fix:
https://github.com/oscar-system/Oscar.jl/actions/runs/12946690999/job/36111735653?pr=4444#step:8:746

Can you please look into it when you have the time (most likely not during the currently ongoing polymake developer meeting)? Many thanks in advance!