fix: fix load_structure method with loading Path object.

[stevenhua0320]

@sbillinge ready to review. Note that I do miss this one in the deprecation process, so in the meanwhile of fix it I also deprecate the method here. Since it originally does not have a test for reading Path object, I add a test at the end.
Closes #124

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.20%. Comparing base (991889b) to head (4f7656c).
⚠️ Report is 5 commits behind head on v3.4.0.

Additional details and impacted files

@@            Coverage Diff             @@
##           v3.4.0     #185      +/-   ##
==========================================
+ Coverage   99.17%   99.20%   +0.02%     
==========================================
  Files          15       15              
  Lines        2439     2501      +62     
==========================================
+ Hits         2419     2481      +62     
  Misses         20       20              

Files with missing lines	Coverage Δ
tests/test_loadstructure.py	98.07% <100.00%> (+0.57%)	⬆️

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