Slow import times

ScippNeutron is taking a long time to import. It seems related to imports of plotting libraries.

- Avoid these imports, either directly or by not importing the problematic submodules (`masking` might be the or one of the offenders).
- Consider SPEC 1 scipp/scipp#3567 as a potential solution.

Note: You can profile imports using
```bash
python -X importtime -c 'import scippneutron'
```