improve handling of unplaced sequences

[matsen]

Have all the unplaced reads as a single pquery.

• When we classify those, they get NULL
• When we make a mass distribution out of them, a warning is emitted.
• Guppy filter without any arguments throws them out.

[matsen]

Even though we shouldn't be getting these anymore, I think this ticket is still alive. Definitely don't want them passing by unnoticed.