Make SVD::solve return an Option insetad of panicking.

[quadrupleslap]

In svd.rs:

/// Returns None if the singular vectors U and V have not been computed.

But it panics, and it actually can't return None because it doesn't return an Option. I don't know whether the comment or the code needs to change.

[sebcrozet]

While this is a breaking change, it is the code that should change to give the user a chance to handle the None case. That means .recompose() and .pseudo_inverse() should return None as well.

[sebcrozet]

Fixed by #441.