import back LeastSquares into eigen2support. Pass most of eigen2's 'regression' test, except for regression_4 which is about complex numbers.

Eigen/LeastSquares

@@ -0,0 +1,31 @@
+#ifndef EIGEN_REGRESSION_MODULE_H
+#define EIGEN_REGRESSION_MODULE_H
+
+#ifndef EIGEN2_SUPPORT
+#error LeastSquares is only available in Eigen2 support mode (define EIGEN2_SUPPORT)
+#endif
+
+#include "Core"
+
+#include "src/Core/util/DisableMSVCWarnings.h"
+
+#include "Eigenvalues"
+#include "Geometry"
+
+namespace Eigen {
+
+/** \defgroup LeastSquares_Module LeastSquares module
+  * This module provides linear regression and related features.
+  *
+  * \code
+  * #include <Eigen/LeastSquares>
+  * \endcode
+  */
+
+#include "src/Eigen2Support/LeastSquares.h"
+
+} // namespace Eigen
+
+#include "src/Core/util/EnableMSVCWarnings.h"
+
+#endif // EIGEN_REGRESSION_MODULE_H