Finish metadata import and export mappings

[soininen]

Metadata import doesn't work after import_mappings rewrite as the work was never finished on them. The mapping classes (ObjectMetadataMapping, ParameterValueMetadataMapping etc.) exist but don't do anything useful. This work should be brought to conclusion.

As metadata comes in name-value pairs, maybe we want to split the mappings accordingly into XMetadataNameMapping and XMetadataValueMapping on the same go. With the original implementation, users had to write JSON in their input files to get name-value pairs properly imported. (Note -- work is already going to this direction in a branch)

We are also missing export mappings for metadata. This should be implemented in parallel with the import mappings.

[jkiviluo]

@soininen Has this issue gone stale and old?

[soininen]

Has this issue gone stale and old?

As far as I can tell, metadata import/export implementations are still missing in the entity branch. This issue might be old but it is not stale.