Store original image downsample in LabeledImageServer

[alanocallaghan]

Continuation of changes discussed in #25

Previously all labeledservers had downsample of 1, meaning they effectively lost any concept of the original image's resolution. This lead to an inconvenient API for the most common use cases (matched use of similar labelled and original images).

This means that running

ls = LabeledImageServer(original_server.metadata, features, downsample=2)
ls.read_region(downsample=1)

will now return an image of the size of original_server, rather than downsampled by 2.

It also seems to mean that region requests are resized to the labeledserver's downsampled coordinate space, without needing to run region.downsample_region() on every request.

[alanocallaghan]

Worth checking the tests I changed/added :)

[Rylern]

I think it would be great if you could add a comment regarding how the new downsample behaviour works (in the docstring of the LabeledImageServer class or in its constructor). Just something to say the LabeledImageServer has the same size as the original server and level 0 can have a downsample > 1

[alanocallaghan]

Great point, thanks! I will also make this clear in the notebooks before merging

[alanocallaghan]

Can you have a quick look over the comment/s and see if it's clear?

[Rylern]

It's clear, thanks