Fix ESSI output time axis and coordinate offsets

[houjun]

Summary

• write ESSI Time as a floating-point axis with one entry per motion step
• fix flattened ESSI coordinate write offsets so nonzero rank offsets target the correct slice
• add serial regression tests for the ESSI helper behavior

Testing

• /global/cfs/cdirs/m3354/perl/conda_eqsim/bin/python -m unittest tests.test_convert_cli tests.test_convert_helpers
• MPLCONFIGDIR=/tmp/mpl_convert_review /global/cfs/cdirs/m3354/perl/conda_eqsim/bin/python convert.py -h5 template/h5NodeCrds.h5 --ssi tests/data/small.ssi -c template/motion_setting.csv -P /tmp/convert_review_run_after_fix

[Copilot]

Pull request overview

This PR fixes ESSI HDF5 output consistency by writing a floating-point Time axis with one entry per motion step and correcting the per-rank coordinate slice offsets when writing flattened ESSI Coordinates. It also adds regression tests to lock in the helper behavior.

Changes:

• Update ESSI Time dataset generation to float64 with length nstep (matching motion datasets).
• Fix flattened ESSI Coordinates write ranges by scaling node offsets by the XYZ width (3).
• Add serial unit tests covering the new helper and ESSI Time axis behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
convert.py	Fixes ESSI Time axis creation and corrects flattened coordinate slice offsets via get_flat_coord_range.
tests/test_convert_helpers.py	Adds regression tests for get_flat_coord_range and ESSI Time axis shape/dtype/values.

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