Address some of the comments on CGNR patch

- Rename BlockDiagonalPreconditioner to BlockJacobiPreconditioner
- Include the diagonal in the block jacobi preconditioner.
- Better flag help for eta.
- Enable test for CGNR
- Rename CONJUGATE_GRADIENTS to CGNR.
- etc.
diff --git a/internal/ceres/types.cc b/internal/ceres/types.cc
index 55910b0..860f8a4 100644
--- a/internal/ceres/types.cc
+++ b/internal/ceres/types.cc
@@ -42,7 +42,7 @@
     CASESTR(DENSE_SCHUR);
     CASESTR(SPARSE_SCHUR);
     CASESTR(ITERATIVE_SCHUR);
-    CASESTR(CONJUGATE_GRADIENTS);
+    CASESTR(CGNR);
     default:
       return "UNKNOWN";
   }