feat: Implement iterator support for k-d trees

[stephenswat]

Since the elements in our k-d tree implementation are stored consecutively in memory, it is trivial (and performant) to provide iterator-based access to them, such that it becomes easier to loop over the elements of the tree in a flat fashion. Of course, the order of the elements will be somewhat unpredictable.

[codecov]

Codecov Report

Merging #1158 (fbc2536) into main (78c8f4a) will not change coverage.
The diff coverage is n/a.

❗ Current head fbc2536 differs from pull request most recent head 4ae157e. Consider uploading reports for the commit 4ae157e to get more accurate results

@@           Coverage Diff           @@
##             main    #1158   +/-   ##
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  Coverage   47.90%   47.90%           
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  Files         359      359           
  Lines       18491    18491           
  Branches     8723     8723           
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  Hits         8859     8859           
  Misses       3603     3603           
  Partials     6029     6029           

Impacted Files	Coverage Δ
Core/include/Acts/Utilities/KDTree.hpp	56.30% <ø> (ø)

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[paulgessinger]

Good idea!