Prevent Clause import rejection when mapped elsewhere.

Per [70849330](https://www.pivotaltracker.com/story/show/70849330).  Clause imports were being blocked with the message "Section code is already used", indicating that they were following a branch of the code (line 19 as of this commit) in `SectionRowConverter` that rejects the import if the clause/section is already part of another directive.  This check, however, should only be performed for Sections; Clauses should be mappable to many directives.

Fix is to override the `setup_object` behavior in the directive class of `SectionRowConverter` that handles clauses and have that method skip this check.