add "metal" Unit for Lammps

[thangckt]

PR Summary:

add "metal" Unit for Lammps data file

PR Checklist

---

• [x] Includes appropriate unit test(s)
• [x] Appropriate docstring(s) are added/updated
• [x] Code is (approximately) PEP8 compliant
• [x] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Patch coverage: 71.42% and project coverage change: -0.09 :warning:

Comparison is base (7941ba4) 89.37% compared to head (bed7ec2) 89.29%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1098 +/- ## ========================================== - Coverage 89.37% 89.29% -0.09% ========================================== Files 61 61 Lines 6164 6173 +9 ========================================== + Hits 5509 5512 +3 - Misses 655 661 +6 ``` | [Impacted Files](https://codecov.io/gh/mosdef-hub/mbuild/pull/1098?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub) | Coverage Δ | | |---|---|---| | [mbuild/formats/lammpsdata.py](https://codecov.io/gh/mosdef-hub/mbuild/pull/1098?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub#diff-bWJ1aWxkL2Zvcm1hdHMvbGFtbXBzZGF0YS5weQ==) | `93.15% <71.42%> (-1.22%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub)

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[daico007]

Also, I think it worthwhile to add some unit test for the new unit_styles (if you have one available).

[daico007]

Will merge after tests finished running

[thangckt]

Dear Prof. @daico007

can you help me with a letter of recommendation? I will very much appreciate for that

Thank you so much.

[daico007]

Hi @thangckt, I am currently still a PhD student so I don't know if I have the credential you're looking for, but I would be happy to help in any way that I can.

[thangckt]

@daico007 can I reach you on some message platforms? my fb: thangckt5