Not crucial: WS widget does not work for negative slit positions

The wiggler spectrum on a screen does not work when the slit position is negative (neither horizontal or vertical axis). It is not a serious bug since we can just use positive values and assume symmetry. Same bug for XOPPY for OASYS 2 :

<img width="805" height="456" alt="Image" src="https://github.com/user-attachments/assets/513f4078-7dab-40ee-a1c5-02a949170afe" />

 Cheers, J