A simple grid is one which only has one layer, that is, the contents of
the objects are the scalar elements (and not other array-like objects).
Both array and foreign-array are simple grids. The function
make-simple-grid will create a simple grid.
It takes the optional keyword arguments (default): grid-type
(*default-grid-type*), dimensions
(*default-dimensions*), element-type
(*default-element-type*), initial-element,
initial-contents.