Implement A := c*A+I method in MatrixHandler

[quantumsteve]

Description

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documentation, enhancement, etc.). Please also include relevant motivation and
context. List any dependencies that are required for this change.

This is part of #393.

Proposed changes

Describe how your changes here address the issue and why the proposed changes
should be accepted.

This adds a new member function MatrixHandler::scaleAddI to compute A := c*A+I. The sparsity pattern (row pointers and column indices) are stored in LinAlgWorkspaceCpu and reused by future calculations. One must check if the diagonal element is already nonzero and add 1 or add a new nonzero element to the matrix.

The test verifies that the sum of rows is 1 larger than the sum of the original row scaled by c.

Checklist

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• All tests pass. Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

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