Generalize BlockIndexVector

[mtfishman]

Generalize BlockIndexVector (which itself is a generalization of BlockIndexRange for representing a subslice of a block to non-ranges) to n-dimensions and also to non-integer indices, which is helpful for generalized indexing used in KroneckerArrays.jl.

[codecov]

Codecov Report

Attention: Patch coverage is 22.80702% with 44 lines in your changes missing coverage. Please review.

Project coverage is 74.86%. Comparing base (08a4d38) to head (8b4d694).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/BlockArraysExtensions/blockedunitrange.jl	21.42%	44 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
- Coverage   76.49%   74.86%   -1.64%     
==========================================
  Files          36       36              
  Lines        1923     1973      +50     
==========================================
+ Hits         1471     1477       +6     
- Misses        452      496      +44     

Flag	Coverage Δ
docs	7.40% <0.00%> (?)

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