Added copy assignment operators to all matrices

src/TNL/Matrices/CSR.h

src/TNL/Matrices/CSR_impl.h

src/TNL/Matrices/ChunkedEllpack.h

src/TNL/Matrices/ChunkedEllpack_impl.h

src/TNL/Matrices/Dense.h

src/TNL/Matrices/Dense_impl.h

src/TNL/Matrices/Ellpack.h

src/TNL/Matrices/Ellpack_impl.h

src/TNL/Matrices/Multidiagonal.h

src/TNL/Matrices/Multidiagonal_impl.h

src/TNL/Matrices/SlicedEllpack.h

src/TNL/Matrices/SlicedEllpack_impl.h

src/TNL/Matrices/Tridiagonal.h

src/TNL/Matrices/Tridiagonal_impl.h

src/UnitTests/CMakeLists.txt

src/UnitTests/Matrices/CMakeLists.txt

src/UnitTests/Matrices/SparseMatrixCopyTest.cpp

src/UnitTests/Matrices/SparseMatrixCopyTest.cu

src/UnitTests/Matrices/SparseMatrixCopyTest.h