populate metadata: Allow no Parent column

[will-moore]

If I have an Image column with Image IDs, but I don't have any Parent column (E.g. Dataset Name or Dataset, currently the populate metadata script fails with:
'Unable to locate Parent column in Row: %r' % row simply because the script wants to check that the Image IDs are valid within a specified Dataset.
This seems too strict in many cases where I am already happy that the Image IDs are valid.

I would prefer that if no Parent column is found, we simply accept that the Image ID is valid.

Instead of

        if images_by_id is None:
            raise MetadataError(
                'Unable to locate Parent column in Row: %r' % row
            )

we simply do:

        if images_by_id is None:
            return long(value)