Add support for outputting related data for a feature in a "single" template. Relations can be specified in restcfg.json under the "GET" method configuration of a template adapter configuration. Relations specify:

 - The Feature Source / Feature Class of the related data
 - A key mapping between the source identity properties and related feature class properties

Under a single template, related data can be access through the $related variable. Data Iterators can be accessed by calling $related->GetRelation("RELATION_NAME"), the relation name being the name of the relation defined in restcfg.json. Related data iterators behave the same as the $model in a "many" template. To guard against bogus relation names in a graceful manner, calling $related->GetRelation() with an invalid relation name will return a null iterator that will immediately terminate when looped within a template.