[MatrixAccumulator] adds 6x6 matrix handling

[EulalieCoevoet]

Needed for sofa-framework/BeamAdapter#115

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[alxbilger]

You can also add the support of 6x6 matrices in DerivativeMatrix::DerivativeElement. Right now, it uses the templated method which calls matAdd (also template). But if you add

void operator+=(const sofa::type::Mat<6, 6, float> & value) const { mat->add(row, col, value); }
void operator+=(const sofa::type::Mat<6, 6, double>& value) const { mat->add(row, col, value); }

it will call the add method you just introduced

[alxbilger]

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