The direction. Possible values are:
- 'x': equivalent to (1, 0, 0)
- 'y': euqivalent to (0, 1, 0)
- 'z': equivalent to (0, 0, 1)
- None: euqivalent to (0, 0, 0)
- an iterable (x, y, z) is returned unchanged.
Returns:
x, y, z : array-like
The direction vector. This is either a numpy.array or zdir itself if
zdir is already a length-3 iterable.