1. Remove constant_sparsity from LinearSolver::Options. It introduces
unnecessarily complexity in the structure of linear solvers and preconditioners.
This is the first step towards cleaning up the Preconditioner interface.

2. Minor tweaks and cleanups to the various linear solvers.
diff --git a/internal/ceres/dense_qr_solver.cc b/internal/ceres/dense_qr_solver.cc
index 3285054..21c5fb5 100644
--- a/internal/ceres/dense_qr_solver.cc
+++ b/internal/ceres/dense_qr_solver.cc
@@ -62,7 +62,7 @@
   }
 
   // rhs = [b;0] to account for the additional rows in the lhs.
-  Vector rhs(num_rows + ((per_solve_options.D !=NULL) ? num_cols : 0));
+  Vector rhs(num_rows + ((per_solve_options.D != NULL) ? num_cols : 0));
   rhs.setZero();
   rhs.head(num_rows) = ConstVectorRef(b, num_rows);