Updates for TensorKit v0.15

[lkdvos]

This PR has a bunch of updates for TensorKit v0.15, which also uncovered some missing functionality and features that I immediately fixed.

In particular, that means a bunch of functions for SumSpace, as well as an improved blocks iterator and block implementation.

[codecov]

Codecov Report

❌ Patch coverage is 80.99174% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.11%. Comparing base (5cd66c0) to head (e8e1673).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/linalg/factorizations.jl	86.30%	10 Missing ⚠️
...c/tensors/abstractblocktensor/abstracttensormap.jl	75.86%	7 Missing ⚠️
src/vectorspaces/sumspace.jl	54.54%	5 Missing ⚠️
src/tensors/abstractblocktensor/abstractarray.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   55.89%   53.11%   -2.78%     
==========================================
  Files          18       17       -1     
  Lines        1426     1363      -63     
==========================================
- Hits          797      724      -73     
- Misses        629      639      +10     

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