Support allocating the output beforehand

[sanderdemeyer]

Would it be possible to support functions like

qr!(A, (Q, R)), svd!(A, (U, S, V)), eigen!(A, (D, V)),

where we have allocated the matrices where the input will be stored already, and e.g. qr! copies the result there?

[andreasnoack]

Notice that the ! methods mostly avoid allocation by reusing the input storage so what you are requesting is essentially the same as qr!(copyto!(Aout, Ain)).