Added validation rule to ensure that there are 0 or 1 instance of the METS agent element, and ensure that the METS agent element has the role of "IPOWNER." This is how we prescribe encoding the ownerId in METS. In the next version of Fedora we will allow multiple ownerIds, which means multiple agents, and we will also decide how to store the agents in the digital object. Not all agents are the object owners, so we don't want to drop this information if we don't have to. But, currently, we don't have a generalized notion of agents in Fedora. So, we grab a selected type of agent (IPOWNER) for the object ownerId for now.