Produces a 2D grid with points arranged in a hexagonal lattice.
The grid is centered on the origin and scaled so the dimension (X or Y) with largest number of points ranges from -1 to 1.
grid = gtm_hxg(xDim, yDim)
xDim, yDim - number of points along the X and Y dimensions, respectively; must be >=2.
grid - a (xDim*yDim)-by-2 matrix of grid points with the first point being the top-left corner
and subsequent points following columnwise.