Ignore dummy objects when guessing ndim in `utils.tracks_to_napari`

[p-j-smith]

Fixes #362

• Ensure the dimensionality of a napari tracks layer doesn't change if data is exported to hdf and re-loaded
• this is done by ignoring dummy objects when guessing ndim in btrack.utils.tracks_to_napari, as they have non-zero z values even for 2D volumes (see https://github.com/quantumjot/btrack/issues/362#issuecomment-1665523967_)

[deprecated-napari-hub-preview-bot[bot]]

Preview page for your plugin is ready here: https://preview.napari-hub.org/quantumjot/btrack/365 Created: 2023-08-04T13:06:07.579659

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ee54546) 85.73% compared to head (4ca83af) 85.73%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #365 +/- ## ======================================= Coverage 85.73% 85.73% ======================================= Files 30 30 Lines 2152 2152 Branches 394 394 ======================================= Hits 1845 1845 Misses 226 226 Partials 81 81 ``` | [Files Changed](https://app.codecov.io/gh/quantumjot/btrack/pull/365?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alan+R+Lowe) | Coverage Δ | | |---|---|---| | [btrack/utils.py](https://app.codecov.io/gh/quantumjot/btrack/pull/365?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alan+R+Lowe#diff-YnRyYWNrL3V0aWxzLnB5) | `85.00% <100.00%> (ø)` | |

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