provide a fallback for retrieving subterps

This release provides a fallback method for retrieving subterps; if fetching e.g. 1005-Interp fails to get subterps, it tries e.g. 1005-Subpart-A-Interp. This is useful for situations where there are multiple subparts each with their own subterps. It should not change the operation of any existing regs because it's purely a fallback for when the first method returns nothing.