Handling morphologies for network models

Migrated from GitHub issue HumanBrainProject/hbp-validation-framework#271, opened by @shailesh on 2020-06-29.

Network models should be allowed to specify one or more links to morphologies. These can be all the morphologies used in the network or a respresentative sample. The APIs should accept a list of such URLs.

Currently, the KG can’t handle morphologies for network models. So this needs to be handled sometime in the future.