Modify Zarr sink addTile to allow creation of new axes

[annehaley]

@manthey identified a use case for the Zarr sink addTile function that is not currently supported:

ts = large_image.new()
ts.addTile(np.zeros((100, 100, 1)), x=0, y=0, z=0, z_value=1)
ts.addTile(np.zeros((100, 100, 1)), x=0, y=0, z=1, z_value=3.2)
ts.addTile(np.zeros((100, 100, 1)), x=0, y=0, z=2, z_value=6.4)
ts.addTile(np.zeros((100, 100, 1)), x=0, y=0, z=2, z_value=6.3, c=1)

This raises an exception new shape must have the same number of dimensions. The addTile function should be modified to support the addition of new axes after the first call.

Originally posted by @manthey in #1625 (comment)