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/system_test.cc b/internal/ceres/system_test.cc
index 01573ac..a9a6747 100644
--- a/internal/ceres/system_test.cc
+++ b/internal/ceres/system_test.cc
@@ -476,6 +476,8 @@
   CONFIGURE(DENSE_SCHUR,            USER,    IDENTITY, 1);
   CONFIGURE(DENSE_SCHUR,            SCHUR,   IDENTITY, 1);
 
+  CONFIGURE(CGNR,                   USER,    JACOBI, 1);
+
   CONFIGURE(ITERATIVE_SCHUR,        USER,    JACOBI, 1);
 
 #ifndef CERES_NO_SUITESPARSE