Work with lower triangle instead of strict lower triangle and diagonal

[geoffroyleconte]

Is it possible to change the algorithm so that the user can work with a lower triangular matrix without having to call tril(A, -1)? I find this not very convenient since most factorization algorithms require the lower or upper triangle (with the diagonal). We could still allocate adiag = diag(A) if needed. @dpo I can try to make these changes if you agree.