Define TensorAlgebra.matricize to return SparseArrays.SparseMatrixCSC

[mtfishman]

This overloads TensorAlgebra.matricize through a package extension to return SparseArrays.SparseMatrixCSC, and defines some more converters to and from SparseArrays.jl and SparseArraysBase.jl objects.

This should make matrix multiplication faster, and will make it easier to define tensor factorizations in terms of SparseMatrixCSC matrix factorizations (though that will require adding support for sparse matrices in MatrixAlgebraKit.jl, see ITensor/TensorAlgebra.jl#21).

This also switches over to using the new [workspace] feature in the Project.toml, see https://pkgdocs.julialang.org/dev/creating-packages/#Recommended-approach:-Using-workspaces-with-test/Project.toml.

As a demonstration:

using SparseArraysBase: sparsezeros
using TensorAlgebra: contract, matricize
a = sparsezeros(2, 2, 2)
a[1, 1, 1] = 1
a[1, 2, 1] = 2
m = matricize(a, (1, 3), (2,))
b, l = contract(a, ("i", "j", "k"), a, ("j", "k", "l"))

results in:

julia> a
2×2×2 SparseArrayDOK{Float64, 3, FillArrays.Zeros{Float64, 3, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}, Base.OneTo{Int64}}}}:
[:, :, 1] =
 1.0  2.0
  ⋅    ⋅ 

[:, :, 2] =
  ⋅    ⋅ 
  ⋅    ⋅ 

julia> m
4×2 SparseMatrixCSC{Float64, Int64} with 2 stored entries:
 1.0  2.0
  ⋅    ⋅ 
  ⋅    ⋅ 
  ⋅    ⋅ 

julia> b
2×2 SparseMatrixDOK{Float64, FillArrays.Zeros{Float64, 2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}}:
 1.0   ⋅ 
  ⋅    ⋅ 

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.47%. Comparing base (78dece8) to head (840095c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sparsearrays.jl	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
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+ Coverage   75.93%   77.47%   +1.54%     
==========================================
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  Lines         698      728      +30     
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