[Yasp] StructuredGridFactory specialization should return `unique_ptr`

Ansgar Burchardtrequested to merge
bugfix/structuredgridfactory-for-yasp-should-return-unique_ptr into master

f96b091d changed the StructuredGridFactory to return a unique_ptr instead of a shared_ptr. This patch implements the same change for the Yasp's specialization of the StructuredGridFactory.

Merge request reports