Check Inputs to Trajectory.

[CompRhys]

Summary

Checks the inputs to Trajectory following an issue encountered where get_positions() method can return more positions than sites depending on filters.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

[janosh]

thanks @CompRhys! 👍

[janosh]

for later reference, considered using np.atleast3d to normalize user input with insufficient dimensions but unfortunately when passed a 2d array, atleast3d inserts a singleton dim at the end rather than the start so probably more hassle than it's worth