Improve icat testability

This PR split the cooking of the icat metadata and the way to push the result.

This improve the readability and will allow to check the result in the future without triggering icat.

• _send_icat_metadata is not supposed to be overwritten
• _get_icat_metadata can be overrided and returns an intermediate dict containing the whole icat metadata

!142 (merged) have to be merged first.