Fix bug with identifier format

Our identifier are padded to be three chars long − eg '001'.
We need to pad the identifier in the dictionary.
Add matching from identifier to place category

Since we have an index mapping the identifiers to places,
we want to use that for categorisation purposes.

This one is a funny one. The index_converter takes the index provided
by the archives and provides us with "identifier --> place".
On the other hand, using the usual manual mapping way,
we have a 'place --> category' mapping along with the other mappings
(ie in the mapper data structure, which is used in post-processing).
And for various reasons, we want to match all that in the post_processing!

Enter match_identifier_to_categories, which is called the usual way by
post_process_collection. It thus drags along in the mapper the second mapping.
We retrieve from the index module the first mapping.
And we just have to glue everything together.
Add Places to metadata_retrieving
Add categorisation statistics display.