BlockTensorKit.oplus renamed to BlockTensorKit.boxplus

[lkdvos]

This PR renames the BlockTensorKit.oplus to become BlockTensorKit.boxplus. The main reason is to avoid the naming conflict with TensorKit.oplus, which is both confusing and annoying to work around.

[codecov]

Codecov Report

❌ Patch coverage is 64.10256% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.43%. Comparing base (3959823) to head (7a7725a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vectorspaces/sumspace.jl	47.82%	12 Missing ⚠️
src/linalg/factorizations.jl	93.33%	1 Missing ⚠️
src/vectorspaces/sumspaceindices.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
- Coverage   55.70%   55.43%   -0.28%     
==========================================
  Files          17       17              
  Lines        1463     1463              
==========================================
- Hits          815      811       -4     
- Misses        648      652       +4     

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