287 include parent child relationships from objects already in the inventory

[LucasZimm]

Entities:

• changed the add_relationship function to accept dictionarys
• added a function to get a unique code for a dictionary

Parser:

• added feature to create relationships with OpenBIS existing objects

Fixed:

• Last merge removed the @Property from dataset attribute in object_type to decrease redundancy. This resulted in a small attribute change "dataset -> datasets"