Matrix addition

[geofjamg]

Signed-off-by: Geoffroy Jamgotchian geoffroy.jamgotchian@gmail.com

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• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest No

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) feature

What is the new behavior (if this is a feature change)? We can add 2 sparse matrices

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

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