Xpetra: add getLocalGraph() method to CrsGraph

Created by: lucbv

@trilinos/xpetra

Expectations

The Xpetra::CrsGraph should expose an interface to getLocalGraph() when Tpetra is the underlying algebra package, this will avoid awkward transformation of Kokkos:View into Teaches::ArrayRCP when trying to set all local graph values at once.

Current Behavior

Currently getLocalGraph() is not implemented so the only way to set all the values in a Xpetra::CrstGraph is to use insertLocalIndices() multiple times, using a Teaches::ArrayView to store the indices which is awkward for on device kernels that attempt to fill the local CrsGraph...

Motivation and Context

This prevents good refactor of code in MueLu when the graph structure is computed at once using direct access to pointers stored by the graph.

Definition of Done

The interface to getLocalGraph() is exposed to users.

Possible Solution

Creating a new stub in Xpetra_CrsGraph.hpp guarded by HAVE_XPETRA_KOKKOS_REFACTOR and HAVE_XPETRA_TPETRA.