Fairly urgent: decide on convention for diagonal in DiagonalOperator

Recently I removed the last remaining uses of the bare keyword in some methods of DiagonalOperator, and this change has now been merged into the nightly branch. The code now behaves as if bare=False, which was the default before.

However Torsten argues that it would be more natural to behave as if bare=True.

Both is fine with me, but we need to decide very quickly, because the first people have started adapting to nightly and will be unhappy if they have to change their codes again!