Add changelog file.

Author: wo80

CHANGELOG.md

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+### Version 4.2.0 - 2024-09-15
+
+* Make `SymbolicColumnStorage` class public and update `StronglyConnectedComponents` and `DulmageMendelsohn` decomposition accordingly.
+
+### Version 4.1.0 - 2024-06-14
+
+* Add overload for creating a sparse matrix from an enumerable of `ValueTuple`.
+* Add matrix `EnumerateIndexedAsValueTuples()` to enumerate entries as `ValueTuple`.
+
+### Version 4.0.0 - 2024-04-03
+
+The major version change is due to the removal of obsolete methods in the `Converter` class. Visibility of that class was changed from public to internal. In case those obsolete methods were still used, please switch to the static conversion methods provided by the `SparseMatrix` class.
+
+Additional changes:
+
+* Add helper method `Helper.ValidateStorage(...)` to validate the structure of a sparse matrix.
+* Update to `GetHashCode()` method of `CompressedColumnStorage` class.
+* Improvements to documentation.
+
+### Version 3.8.1 - 2023-11-15
+
+* Add overloads for permutation `Invert()` and `IsValid()` methods taking the permutation length as argument.
+
+### Version 3.8.0 - 2023-05-20
+
+* Add overloads for the factorization `Solve()` methods taking `Span<T>` as argument. Note that this introduces a dependency on `System.Memory` for the netstandard2.0 assembly.
+
+### Version 3.7.0 - 2022-05-04
+
+* Add sparse matrix `OfDiagonals` static method (similar to MATLAB spdiags).
+
+### Version 3.6.0 - 2021-11-25
+
+* Remove .NET 4.5 target framework, upgrade .NET 5.0 to 6.0.
+* Add constructor that takes explicit non-zeros count to `CoordinateStorage` class.
+
+### Version 3.5.0 - 2021-01-14
+
+* Remove .NET 4.0 target framework, add .NET 5.0.
+
+### Version 3.4.9 - 2020-11-06
+
+* Add `CoordinateStorage` constructor that uses existing storage arrays.
+* Convert `CoordinateStorage` to sparse matrix in place.
+
+### Version 3.4.7 - 2020-08-28
+
+* BREAKING: make `SparseLDL` constructor private (use static create methods instead).
+* Add complex version of `SparseLDL`.
+* Add `matrix.EnumerateIndexed(action)` overload.
+
+### Version 3.4.6 - 2020-07-21
+
+* Add `SolveTranspose` method for `SparseQR`.
+
+### Version 3.4.5 - 2020-06-11
+
+This release introduces the static `SparseMatrix.AutoTrimStorage` option, which enables control over hidden memory allocations in matrix addition and multiplication. By default, the matrix storage will be resized to exactly fit the non-zeros count, which involves new memory allocations. If you want to avoid this, set `AutoTrimStorage` to `false`.
+
+Additional changes:
+	
+* Add public helper methods `Helper.TrimStrorage(...)` and `Helper.SortIndices(...)`
+* Add `DenseMatrix.OfJaggedArray(...)`
+
+### Version 3.4.3 - 2020-05-25
+
+* Add a sparse matrix multiplication overload that accepts the result matrix as a parameter.
+
+### Version 3.4.2 - 2020-05-13
+
+* Make CSparse.NET CLS compliant
+* Mark public methods of Converter class as obsolete
+
+### Version 3.4.1 - 2019-10-02
+
+* Improved validation of matrix constructor arguments
+* Fixes an issue with `CoordinateStorage` throwing `IndexOutOfRangeException` (introduced in v3.4.0)
+
+### Version 3.4.0 - 2019-09-15
+
+* Parallel dense and sparse matrix multiplication (by Andreas Girgensohn)
+* General performance improvements for sparse matrix addition and multiplication
+
+### Version 3.3.0 - 2019-04-29
+
+* Support more target frameworks (including netstandard2.0).
+* Public access to members of Dulmage-Mendelsohn decomposition.
+* Compute strongly connected components.
+
+### Version 3.2.3 - 2018-11-30
+
+* Added matrix creation helper (e.g. call `SparseMatrix.OfIndexed(s)` to convert coordinate storage).
+
+### Version 3.2.2 - 2018-10-12
+
+* Added MatrixMarket writer.
+* BREAKING: make `IProgress interface` compatible with .NET 4.5.
+
+### Version 3.2.1 - 2018-09-17
+
+### Version 3.2.0 - 2018-03-09
+
+* Added new `DenseMatrix` type.
+* BREAKING: removed deprecated `CompressedColumnStorage` type.
+* BREAKING: removed deprecated `matrix.Norm(int)` method.
+
+### Version 3.1.10 - 2018-03-06
+
+* Rename `CompressedColumnStorage` to `SparseMatrix`.
+* BREAKING: `matrix.Multiply(x, y)` overwrites y (instead of update).
+* BREAKING: sparse matrix `PermuteColumns` returns a new matrix (instead of update).
+
+### Version 3.1.9 - 2017-01-06
+
+* BREAKING: use static `Create` methods (e.g. `SparseLU.Create(...)`) instead of constructors.
+
+### Version 3.1.4 - 2015-09-19
+
+* Initial release of CSparse.NET (based on Tim Davis CSparse version 3.1.4)