Allow 'xyz_heightmap()'s 'col' argument to be a vector or function (as well as matrix)

[trevorld]

• If col is a matrix treat the same as before

• If col is a vector (of colors) then use them to color cubes by height

  • If fill is TRUE this will color the filled in lower cubes differently by height

• If col is a function then treat it as a palette function using max(z) as its n

[trevorld]

Can use as.integer(cut(z, breaks = n)) to break z into n ranges (for colors)