Fix the type for acquisitions.id under the plate metadata

[sbesson]

Although the type is not specified in https://ngff.openmicroscopy.org/0.4/#plate-md, the snippets use integers for the aqcuisitions identifiers. The well.acquisition metadata is also of integer type to create a reference to the acquisitions objects defined at the plate level.

39237e2 updates the current writer code and adjusts the tests accordingly. Note this PR is currently built on top of #149 which adds a new HCS test with the previous behavior for the id type.

[sbesson]

@melissalinkert I also had a quick look at raw2ometiff to see whether this would require an associated PR but I think plate.acquisitions.id is only consumed in https://github.com/glencoesoftware/raw2ometiff/blob/master/src/main/java/com/glencoesoftware/pyramid/PyramidFromDirectoryWriter.java#L376 and the usage of toString() should work with Integer.
In order to run a minimal test, are the current HEAD of bioformats2raw and raw2ometiff compatible at the moment or is anything required on the raw2ometiff front to consume the 0.5.0-SNAPSHOT changes?

[melissalinkert]

I'm fine with this, but it probably makes sense to update the spec as well to clarify the correct type.

[melissalinkert]

I'd expect data generated by current HEAD of bioformats2raw to work with current HEAD of raw2ometiff. I wouldn't try to update the bioformats2raw dependency in raw2ometiff locally (due to jzarr version differences).

Keep in mind though that https://github.com/glencoesoftware/raw2ometiff/blob/master/src/main/java/com/glencoesoftware/pyramid/PyramidFromDirectoryWriter.java#L376 will only be executed if there is no OME-XML metadata file:

https://github.com/glencoesoftware/raw2ometiff/blob/master/src/main/java/com/glencoesoftware/pyramid/PyramidFromDirectoryWriter.java#L624

[sbesson]

See ome/ngff#120 for the OME-NGFF JSON schema PR specifying the type of acquisition.id