Extract the grid points from a grid, returning columns.
This is useful for e.g. gridding large datasets, in which the first step
might be to use
binMean2D, followed by
a vector holding the x coordinates of grid.
a vector holding the y coordinates of grid.
a matrix holding the grid.
A list containing three vectors:
x, the grid x values,
y, the grid y values, and
grid, the grid values.
1 2 3 4 5 6