[Bugfix] Construct arrays of int from arrays of unsigned int

When cleaning up yasterday, I was mislead by the incode doc
which stated this code is to avoid the deprecation warnings
coming from Yasp construcors.
[Test] Add test cases to the structuredgridfactorytest

... to cover the specialization for
Yaspgrid<dim, EquidistantOffsetCoordinates<ct,dim>>