add Niivue property for mm and vox coordinates

[hanayik]

something like:

// make new Niivue instance in UI code/component
let niivue = new Niivue()

// implement some mouse move event (.e.g. onclick)
// within the callback you could call something like this:

mm = niivue.loc.mm // [mm, mm, mm]

vox = niivue.loc.vox // [vox, vox, vox]

// then set a UI element string to show mm, or vox values to the user.