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

---

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

[daico007]

LGTM! Will merge after all test passes.

[codecov]

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

@@           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	Coverage Δ
mbuild/formats/xyz.py	100.00% <100.00%> (ø)
mbuild/packing.py	95.25% <100.00%> (+0.03%)	⬆️

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[chrisiacovella]

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