Tile sink should accept tiles with no explicit samples dimension

[manthey]

Given a numpy array: data = np.zeros((4, 1040, 1388)) which is channels, y, x, this doesn't work:

sink = large_image.new()
sink.addTile(t, 0, 0, axes=['c', 'y', 'x']
sink.write('sample.tiff')

But this does:

sink = large_image.new()
sink.addTile(t[..., np.newaxis], 0, 0, axes=['c', 'y', 'x', 's']
sink.write('sample.tiff')

If axes are specified explicitly and match the number of axes of the numpy array, this should work.

[manthey]

@annehaley This was reported from a user. We should add a test case for this and make sure the first instance works.