Fix matrix factorizations of block sparse arrays with abstract block type

[mtfishman]

The strategy is to put off determining the algorithm type until right before the blocks are being factorized, instead of up front based on the block type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.40%. Comparing base (33330bd) to head (dea2c79).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   73.97%   76.40%   +2.43%     
==========================================
  Files          36       36              
  Lines        1875     1916      +41     
==========================================
+ Hits         1387     1464      +77     
+ Misses        488      452      -36     

Flag	Coverage Δ
docs	7.63% <0.00%> (-0.17%)	⬇️

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