[Core] Introduce WriteAccessor move constructor

[alxbilger]

It allows to write such code:

sofa::type::vector<sofa::helper::WriteAccessor<InDataMatrixDeriv> > outAccessors;
outAccessors.reserve(dOut.size());
for (auto* out : dOut)
{
    outAccessors.emplace_back(out);
}

[with-all-tests]

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[fredroy]

I wonder what should/will happen if one use the moved thing after the move (std::move and stuff) ? 🤔

[alxbilger]

I wonder what should/will happen if one use the moved thing after the move (std::move and stuff) ? 🤔

According to the unit tests, the accessor stays valid but does not trigger endEdit. Note that clang-tidy triggers a warning in this case: https://clang.llvm.org/extra/clang-tidy/checks/bugprone/use-after-move.html.