Change exception to warning if xy_scale_size metadata is absent

[talonchandler]

Fixes #180.

Also, defaults to 1.0 um if metadata is absent.

[ziw-liu]

We can also modify the info command to catch the error and show 'not available'. The main concern I have is user code depending on the old behavior will silently use the wrong value if it's not executed in an interactive environment.

[talonchandler]

Possibly naive, but is it possible for users to depend on the old behavior if it raises AttributeError when the metadata is absent?

I also think that setting to default 1.0 is the right thing to do here since that's what happens for the ome-zarr files with identity transformations, right?

FWIW this code was introduced ~3 months ago in #118.

[ziw-liu]

is it possible for users to depend on the old behavior if it raises AttributeError when the metadata is absent?

Something like:

try:
    px_size = reader.xy_scale_size
except AttributeError:
    px_size = json.load("some/custom/metadata.txt")["pixel_size"]

I don't know anyone who is doing this, but if the purpose is just to fix #180 there is also no need to break APIs.

[ziw-liu]

Closing in favor of #185 and will be shipped in v0.2.0.

Thanks @talonchandler!