Adding example datasets

[yfukai]

LapTrack has several example datasets. It may be useful for the demonstration to expose them as sample datasets for the plugin.

[haesleinhuepf]

We could also select some example data from here:
https://zenodo.org/search?page=1&size=20&q=trackmate%20example%20data

And code-wise examples can be made available like done here:
https://github.com/haesleinhuepf/napari-process-points-and-surfaces/blob/main/napari_process_points_and_surfaces/__init__.py#L99-L105