[LinearAlgebra] CompressedRowSparseMatrixConstraint: make consistent the serialization/deserialization process

[fredroy]

For now, the serialization of a CSR cannot be used a deserialization as the format are different.
In a nutshell, it is not possible to do:

    std::ostringstream oss;
    oss << outMatrix;

    CSR inMatrix;
    std::istringstream iss(oss.str());
    iss >> inMatrix;

So this PR tries to fix it by changing the output format of the serialization.
+ more concise (faster to serialize, less space)
- much less human-readable

Other/complimentary solutions:
- adding a nice printing (same as the deserialization now)
- dont change the serialization but the deserialization

EDIT:

• restoration of the pretty output in a function (prettyPrint())
• use this print in a specialization for its Qt Widget (so no change of behavior with the Qt GUI)

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

[ci-build][with-all-tests]