Clarification of Coverage Responsibilities

Change this:

. payor Σ 1..* Reference(Organization | Patient | RelatedPerson) Issuer of the policy

TO:

. insurer     Σ     0..1     Reference(Organization)       Issuer of the policy

. payer      0..1     backboneElement

     .party       Σ      0..1    Reference(Organization | Patient | RelatedPerson)    Party accepting financial responsibility for some aspect of the services provided

     .responsibility     0..1     string     Description of the financial responsibility 

     Add invariant that says either AN insurer OR a payer is required, but not both