optimize code for RDA RTs

[briesenberg07]

Schemas, conversion code, prop_sets, etc. could be simplified if we allowed only those Sinopia features which may be used to create well-formed RDA. (Most often or always, that is, do not support Sinopia features which create blank nodes.)

Such features include:

• nested-resource templates (which create blank node values in description sets)
• property templates set as repeatable + ordered, which use blank nodes to order values
• resource-template attribute 'suppressible', which is used to eliminate a blank node when a nested-resource property template uses a nested resource with only one property template, eliminating the bnode and providing the template's property value as the value of the property in the nesting template

[briesenberg07]

Related to issues:

• https://github.com/uwlib-cams/map_storage/issues/44