Slight cleanup and refactor of sliderpack

[oerc0122]

Description of work Generalise sliderpack and reduce manual code for 2 sliders.

Fixes N/A

To test Sliderpack should work as normal.

[MBartkowiakSTFC]

There is a slight functionality change. In the main branch, the sliderpack made it impossible to set heatmap maximum to be lower than minimum:

After the changes, nothing can stop me: