Fix regressions of changes to composite nodes (list, figure).

Note: an image is used as a primitive node now.
Instead a figure is used to pack an image with an caption.

Moreover, the properties to referenced nodes return ids
not nodes. E.g., list.items gives an array of ids.
Fix regressions.
Moved pandoc specific fixture files into a subfolder.
Extracted error definitions to be shared among converters.
Initial NLM importer implementation and test.
Initial support for importing paragraphs from NLM.
Supporting NLM with a single section.
Support nested sections in NLM.
Basic annotation support for NLM.
Not working yet: Figures from NLM.

Needs a merge of the nested_nodes branch.
Merge branch 'nested_nodes' into nlm_converter
Fix basic figure support in NLM Importer.
Add rudimentary support for sub/sup/underline.
Clean up.
Renamed Importer to PandocImporter.

Same with Exporter.
Tackled a cyclic dependency in Lens.

This probably breaks other places, though.