publication field not implemented

Migrated from GitHub issue HumanBrainProject/hbp-validation-framework#288, opened by @shailesh on 2020-10-15.

Neither tests nor models presently have the field named 'publication' implemented. Seems to be a TODO item: https://github.com/HumanBrainProject/hbp-validation-framework/blob/2ba3da37c9f237dc7d2f43600aac0ae07f08cdf9/validation_service_v2/validation_service/data_models.py#L548