cellsize field that updates with reduction along a dimension

[rafaqz]

Cells need to have a size in all dimensions. When a method like mean or sum that reduce a dimension to size 1 is called, the cell size on that dimension should be updated to match - to span the whole range of the original dimension instead of one cell.

ie cellsize *= length(val(dim))

The starting point would remain as is currently - the start of the first cell.

This could be a field of the grid trait type, or not.