In essense, this change removes the depencence on specific eprints fields
having values when performing the XML export.

This expands on the change in 336a27c
("reworked Export plugin so that *any* EPrint field can be mapped if
corresponding sub is found in zzz_datacite_mapping") and removes the change
required in f8a3259 becuase now we run all
maps.

It moves from an 'eprint first' configuration to a 'mapping function first'
which permits:
- Setting default values (like Publisher)
- Choosing the eprint fields data comes from (You have a custom date field? no worries).
- Adding new values to output XML by adding a new mapping function
- Basically, running arbitrary code. But don't do that.

Of course, anyone adding new mappings or overriding existing mappings will need
to handle the resulting validation problems themselves...

This is the export components work required to complete #35; optional changes
to validation would facilitiate the second part but validation can be done via
overrides per site.

Closes: #35