ID of merged Described is always the lowest ID in the input Describeds

When merging `Described`s sets the `OutputContentMerger` will set the ID
of the chosen `Described` to the lowest ID present in its equivalence
set.
determine lowest id from chosen equivalentTo set
Merge pull request #140 from atlasapi/mbst-6435-id-output

MBST-6435 id output