TCTracks.from_simulations_emanuel: optionally, only load a subset

[tovogt]

Changes proposed in this PR:

• Small feature that allows to read a subset of a track data set instead of the whole file. Since the simulation data sets can be quite huge, it's a pain having to read everything, even if you're only interested in a (random) sub-sample.
• Also, fixes a bug where the "time_step" variable was stored as an integer which caused problems when interpolating to sub-hourly resolutions.

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• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
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• Documentation updated
• Tests updated
• Tests passing
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PR Reviewer Checklist

• Read the Contribution Guide
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[emanuel-schmid]

🙌 ready to merge