More on sparspak

[j-fu]

• set default algorithm to sparspak for generic float
• precompile sparspak
• Mention sparspak in README

[codecov[bot]]

Codecov Report

Merging #255 (d224203) into main (c31e460) will increase coverage by 0.86%. The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
+ Coverage   65.58%   66.44%   +0.86%     
==========================================
  Files          12       12              
  Lines         738      757      +19     
==========================================
+ Hits          484      503      +19     
  Misses        254      254              

Impacted Files	Coverage Δ
src/default.jl	48.05% <33.33%> (ø)
src/LinearSolve.jl	95.65% <100.00%> (+20.65%)	:arrow_up:
src/factorization.jl	78.60% <100.00%> (ø)

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