Use Sparspak for general floats and \ for non-GPL

[j-fu]

Sparspak.jl provides support for general floats and is MIT licensed. So we can define \ and ldiv! for ExtendableSparseMatrix, and also as an alternative for the non-GPL case.

[codecov-commenter]

Codecov Report

Base: 85.15% // Head: 83.20% // Decreases project coverage by -1.95% ⚠️

Coverage data is based on head (c0af0ec) compared to base (a4a5bc0).
Patch coverage: 80.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   85.15%   83.20%   -1.96%     
==========================================
  Files          17       18       +1     
  Lines         842     1012     +170     
==========================================
+ Hits          717      842     +125     
- Misses        125      170      +45     

Impacted Files	Coverage Δ
src/ExtendableSparse.jl	100.00% <ø> (ø)
src/sprand.jl	71.17% <64.70%> (-5.16%)	⬇️
src/sparspak.jl	81.81% <66.66%> (-18.19%)	⬇️
src/SparspakCSCInterface.jl	81.87% <81.87%> (ø)
src/extendable.jl	62.99% <100.00%> (-0.72%)	⬇️
src/factorizations.jl	73.33% <100.00%> (ø)
src/sparsematrixlnk.jl	96.79% <0.00%> (-0.65%)	⬇️

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[j-fu]

Hi @bmmogensen, in this PR I changed the architecture for the non-GPL case:

• Defined my own constant USE_GPL_LIBS in order to have a switch for at least some testing
• Provide a Base.\ for ExtendableSparseMatrix relying on Sparspak.jl

May be you can check if this still works for you before I tag a new release, would wait a couple of days.