[poe] Issue: Allow Asset/Party at the Policy level marked as To Be Closed

riannella has just labeled an issue for https://github.com/w3c/poe as 
"To Be Closed":

== Allow Asset/Party at the Policy level ==
In many cases, the Asset (target) and Parties (assigner, assignee) are
 the same for multiple Permissions and Prohibitions in an ODRL Policy.

It maybe a useful change to support the Asset/Party at the Policy 
level, which means that these values all apply to the enclosed 
Perms/Prohibs (and conversely means that the Perms/Prohibs do not 
define these values). 


See https://github.com/w3c/poe/issues/82