feature request: mesh scaling options

[shimwell]

Making use of this super plotter and I'm keen to be able to plot RegularMesh results with a custom scaling factor.

I am also keen to be able to plot with the RegularMesh result normalised by the voxel size.

I would be happy to put in a couple of PRs if the request sounds reasonable

[shimwell]

I have had a go at adding a volume normalisation checkbox over here

I was able to add a checkbox and divide the data by the mesh volume.

I'm currently stuck on the passing of the checkbox state to the function that gets the data to plot to control if the data is normalised or not

[yrrepy]

This would be really useful to have.