Multiple dense linear algebra backends.

1. When a LAPACK implementation is present, then
DENSE_QR, DENSE_NORMAL_CHOLESKY and DENSE_SCHUR
can use it for doing dense linear algebra operations.

2. The user can switch dense linear algebra libraries
by setting Solver::Options::dense_linear_algebra_library_type.

3. Solver::Options::sparse_linear_algebra_library is now
Solver::Options::sparse_linear_algebra_library_type to be consistent
with all the other enums in Solver::Options.

4. Updated documentation as well as Solver::Summary::FullReport
to reflect these changes.

Change-Id: I5ab930bc15e90906b648bc399b551e6bd5d6498f

internal/ceres/iterative_schur_complement_solver.cc

diff --git a/internal/ceres/iterative_schur_complement_solver.cc b/internal/ceres/iterative_schur_complement_solver.cc
index d39d7db..0b1cb89 100644
--- a/internal/ceres/iterative_schur_complement_solver.cc
+++ b/internal/ceres/iterative_schur_complement_solver.cc
@@ -97,8 +97,8 @@
 
   Preconditioner::Options preconditioner_options;
   preconditioner_options.type = options_.preconditioner_type;
-  preconditioner_options.sparse_linear_algebra_library =
-      options_.sparse_linear_algebra_library;
+  preconditioner_options.sparse_linear_algebra_library_type =
+      options_.sparse_linear_algebra_library_type;
   preconditioner_options.num_threads = options_.num_threads;
   preconditioner_options.row_block_size = options_.row_block_size;
   preconditioner_options.e_block_size = options_.e_block_size;