[core.Abstract] add alt_id field
gca-cat: Support deduplication
gca-sort: support identity and alt_id assignment
[core] add Conference|Group
gca-client: add option to fetch a conference
[core.Conference] fix group filter method
[core.Conference] add factory method 'from_data'
[core.Author] add full_name property
[core.Abstract] ctor: take a optinal conference
[core.Abstract] add poster_id property
gca-select: cmd line option to supply conf. info

Can be used to resolve poster_ids for an abstract