Fix issue with kD-tree example not building

[sdhiscocks]

Sometimes the build failed due to example taking too long to run. I've reduced the number of tracks in simulation, and avoided running simulation multiple times.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +4.44 :tada:

Comparison is base (a4df8fd) 90.14% compared to head (35ac949) 94.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #807 +/- ## ========================================== + Coverage 90.14% 94.58% +4.44% ========================================== Files 184 184 Lines 10698 10698 Branches 2176 2176 ========================================== + Hits 9644 10119 +475 + Misses 857 398 -459 + Partials 197 181 -16 ``` | Flag | Coverage Δ | | |---|---|---| | integration | `69.62% <ø> (?)` | | | unittests | `90.14% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl#carryforward-flags-in-the-pull-request-comment) to find out more. [see 31 files with indirect coverage changes](https://app.codecov.io/gh/dstl/Stone-Soup/pull/807/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl)

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