Fix publication_date

[matentzn]

Resolves #293

A property (publication_date) was defined in SSSOM, but was not added to the SSSOM mapping and mapping set data model. Since a very similar property (mapping_date) was already defined, this change is considered a bugfix, rather than an added feature.

[gouttegd]

To be honest I am not convinced it makes sense to have a publication_date at the mapping level, anymore than I am convinced it makes sense to have a mapping_date at the mapping set level.

To me it would make more sense to have only a publication_date at the mapping set level (“when the mapping set was published”) and a mapping_date at the mapping level (“when that particular mapping was asserted”).

But you know SSSOM use cases better than I do, so I assume you know what you’re doing. ;)

[matentzn]

The main use case that needs to be covered is mix'n'match for now (merging multiple mapping sets, losing as little information as possible), but yeah, I get your point. I believe we should try to be as liberal as possible and constrain the model more at a local level if I need to.