MueLu: detect if input matrix is identity
Created by: jhux2
A user recently tried to run MueLu on an identity matrix. For a large enough system, MueLu will coarsen to a zero-by-zero system, which causes problems. There should be a mechanism for detecting if all DOFs are boundary points, i.e., there are zero aggregates. One solution is to throw an exception during Dirichlet detection.