This change is needed for being able to use different thread indices types
without changing Fetchs. Basically decoupling those two areas.
1. This commit removes concrete specialization instantiations of
ThreadIndicesTypes in all of the Fetch's specializations.
2. It also moves the ThreadIndicesType template parameter from the Fetch
struct to a template parameter in their methods Load/Store.
Signed-off-by: Vicente Adolfo Bolea Sanchez <vicente.bolea@kitware.com>
