revised load_xyz function

[chrisiacovella]

PR Summary:

This addresses issue #1139 . Loading of xyz files did not see the prior changes to adding multiple compounds as a single list, rather than one at a time.

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: 100.00% and no project coverage change.

Comparison is base (b4836ae) 87.11% compared to head (3aa8410) 87.12%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1140 +/- ## ======================================= Coverage 87.11% 87.12% ======================================= Files 62 62 Lines 6451 6455 +4 ======================================= + Hits 5620 5624 +4 Misses 831 831 ``` | [Files Changed](https://app.codecov.io/gh/mosdef-hub/mbuild/pull/1140?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/xyz.py](https://app.codecov.io/gh/mosdef-hub/mbuild/pull/1140?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub#diff-bWJ1aWxkL2Zvcm1hdHMveHl6LnB5) | `100.00% <100.00%> (ø)` | | | [mbuild/packing.py](https://app.codecov.io/gh/mosdef-hub/mbuild/pull/1140?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mosdef-hub#diff-bWJ1aWxkL3BhY2tpbmcucHk=) | `95.25% <100.00%> (+0.03%)` | :arrow_up: |

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[chrisiacovella]

All tests successfully passed. Looks like we are good to merge.