Migrate to pandas Copy-on-Write

[lkstrp]

Since this has come up in discussions:

• Pandas is migrating to copy and write behaviour (see here). In pandas>=3.0 this will be the default.
• Most of the problems currently associated with current implementation are already handled by pandas warnings (see docs). But we might want to switch to this at some point before pandas 3.0 arrives, to get all the benefits and let people be more sure about their data references. There's a lot of safety-first .copy()ing going on now, which wouldn't be necessary and would be more clear.