Curves dialogue

[RussellGarwood]

Describe the bug
When a dataset is downsampled in Z the curves dialogue does not appear to show correctly which slices the curve is on, at least on a mac

To Reproduce

1. Open a downsampled .spe file
2. Add a curve, interpolate over multiple slices
3. Look at the slices it is reported to be on in the curves dialogue

Desktop (please complete the following information):

• OS: macOS