2008-09 to 2009-09: Move CustomAttributes transforms to top level

[dgault]

This is a follow up to #142 and should see the unit tests in ome/bioformats#3695 turn green.

The issue appears to have been due to CustomAttributes being able to be located in 4 different locations:

• top level under OME
• under images
• under images/feature
• under dataset

The non top level CustomAttributes were being transformed into XMLAnnotations but embedded in the original element, be it Image or Dataset, this is an invalid location for it and thus it was not being recognised when converted back to an OME-Model object. This PR aims to move all CustomAnnotations to the top OME level before transforming them. If you have multiple CustomAttributes at different levels then you will get multiple XMLAnnotations inside a single StructuredAnnotations.

Fixes #142

[sbesson]

@dgault looks like this PR has been opened from a branch on the ome/ome-model repository itself. Unfortunately, while Travis has been disabled for Pull Requests, there was a remaining webhook that created a failing build on the push event which will I cannot easily remove.

Could you either force push or recreate a PR from your fork?