Add missing convert methods for Cholesky

[timholy]

No description provided.

[dkarrasch]

This is usually done by relying on generic convert methods and overloading the constructors instead. That's also why we then don't need to handle the noop case.

[dkarrasch]

Similar conversion methods seem to be missing for most other factorizations, so this looks like a little "project". ;-)

Also, I didn't write out all the conversions, because they are handled by the inner constructor, once all type parameters are specified.

[timholy]

Thanks @dkarrasch!