Pb :
M() = rhs; // rhs of type RHS
Currenlty does :
M(i,j) = (i==j ? rhs : RHS{})
Changed to
M(i,j) = (i==j ? rhs : RHS{0*rhs})
If RHS is a double, int ... Same result.
If RHS is a matrix, gf, currently the offdiag elements
are default constructed (i.e. of 0 size !).
Which can break operations later (matrix<matrix<double>>)
After change : all elements have the same size !
