MHD Document Responder: patient.identifier chaining

This is a quick article on a requirement of the MHD Document Responder that may be less obvious to some. The specific requirement is related to chained search parameters, like `patient.identifier`;  `source.given` and `source.family`; and `author.given` and `author.family`. These search parameters have a `.` that indicates that one is to search deeper, aka chaining the search value to a different kind of resource and use the results in the primary search.The requirement is hinted at being special with a mention...