mrmath: Behaviour for inequal voxel grids

[Lestropie]

Reported by @EYPierre.

If mrmath is run with multiple input images, and those input images have compatible axis sizes (necessary given it's a voxel-wise operation), then currently the command will simply proceed, using the first input image as the template. Better would be to run the function that checks whether voxel grids overlap in scanner space: if no, issue a warning, compute the mean header transform across the input images, and use that in formation of the output image.