Make Scipp a hard dependency of Plopp

[nvaytet]

See #560 for context and discussion.

Plopp uses Scipp everywhere internally. It makes sense that it should be a hard requirement.
Until now, we had it as a soft dependency because of fears of ending up in a circular dependency issue where plopp needs scipp but scipp needs plopp (because it imports it and adds the .plot() methods to DataArray).

But as long as Plopp just remains a soft dependency of Scipp, solving environments should be fine.

• if you pip install plopp, you would get plopp and scipp (and skip scipp if it is already installed)
• if you pip install scipp, you get only scipp
• if you pip install scipp plopp you get both at once, as expected.

This will also make it easier to keep our conda recipe healthy and in-sync with the pyproject.toml here.

[MridulS]

I'm okay with this too, we just need to be consistent :)