[BlockSparseArrays] Improve the design of block views

[mtfishman]

This fixes a few issues related to block views listed in #1336. The strategy here is to change the design such that calls like @view a[Block(1, 1)] return something like SubArray(a, Block(1, 1)), while before this PR it returned the block of the BlockSparseArray directly, which causes problems when getting views of blocks that aren't stored. There are still issues to work out with that new design but they should be easy to fix.

To summarize, this fixes:

1. Initializing blocks with broadcasting, such as a[Block(2, 2)] .= 1 or a[Block(2, 2)] .= randn(2, 2).
2. Setting elements of blocks with @view(a[Block(1, 1)])[1:1, 1:2] = rand(1, 2) or @views a[Block(1, 1)][1:1, 1:2] .= rand(1, 2).
3. Setting blocks with the syntax a[Block(2), Block(2)] = randn(3, 3) (before only a[Block(2, 2)] = randn(3, 3) worked).
4. Catching trying to set a block with an array of the wrong size, i.e. a[Block(1, 1)] = randn(3, 3) of size(a[Block(1, 1)]) != (3, 3).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.14%. Comparing base (e08e131) to head (d15ae71).
Report is 15 commits behind head on main.

❗ Current head d15ae71 differs from pull request most recent head e948603

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1481      +/-   ##
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+ Coverage   43.65%   44.14%   +0.49%     
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  Files         136      144       +8     
  Lines        8806     9374     +568     
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+ Hits         3844     4138     +294     
- Misses       4962     5236     +274     

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[mtfishman]

@ogauthe merging this now. Please try it out and report any issues you come across.

[emstoudenmire]

Nice to see this