feat: dynamic dataset size units in cli output

[gordonwatts]

Summary

• format dataset sizes in MB, GB, or TB with two decimal places and include a space before the unit
• test dataset listing for MB, GB, and TB cases

Testing

• black servicex/app/datasets.py tests/app/test_datasets.py
• flake8
• pytest

---

https://chatgpt.com/codex/tasks/task_e_68927a4af9e8832094127416bee326ca

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (63669b9) to head (1fbbc10).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #655      +/-   ##
==========================================
+ Coverage   97.19%   97.24%   +0.05%     
==========================================
  Files          29       29              
  Lines        1958     1998      +40     
==========================================
+ Hits         1903     1943      +40     
  Misses         55       55              

Flag	Coverage Δ
unittests	97.24% <100.00%> (+0.05%)	⬆️

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