Given a Lie algebra L, return the direct sum decomposition of L as a sequence of ideals of L whose sum is L and each of which cannot be further decomposed into a direct sum of ideals.

> L := SimpleLieAlgebra("D", 2, RationalField());
> L;
Lie Algebra of dimension 6 with base ring Rational Field
> D := DirectSumDecomposition(L);
> D;
[
    Lie Algebra of dimension 3 with base ring Rational Field,
    Lie Algebra of dimension 3 with base ring Rational Field
]
> Morphism(D[1], L); 
[ 0  1  0  0  0  0]
[ 0  0  1 -1  0  0]
[ 0  0  0  0  1  0]
> Morphism(D[2], L);
[1 0 0 0 0 0]
[0 0 1 1 0 0]
[0 0 0 0 0 1]