Fix issues in `vtkmSlice` and `vtkmAverageToPoints`
- vtkmSlice now works with uniform grid: This path was previously disabled because there was a bug in VTK-m's flying edges implementation that caused crashes. The issue has since been resolved. Addresses: #18856 (closed)
- Fix handling of coordinates field in vtkmAverageToPoints: A recent change in VTK-m starts treating coordinate points as fields. This requires special handling when we don't want to process coordinates. Addresses: #18855 (closed)