Remove cblas calls, use EIGEN_USE_BLAS instead.

[susilehtola]

As requested in #81, this is a pull request that eliminates CBLAS in favor of BLAS support inside Eigen3.

Note that to actually compile the code against the present version of Eigen3, you need to address a bug in Eigen3 by adding a missing include statement

$ hg diff
diff -r 3a08a093adac Eigen/src/Core/products/TriangularMatrixMatrix_BLAS.h
--- a/Eigen/src/Core/products/TriangularMatrixMatrix_BLAS.h     Mon Apr 09 13:29:26 2018 +0200
+++ b/Eigen/src/Core/products/TriangularMatrixMatrix_BLAS.h     Tue Apr 10 15:51:12 2018 +0300
@@ -33,6 +33,8 @@
 #ifndef EIGEN_TRIANGULAR_MATRIX_MATRIX_BLAS_H
 #define EIGEN_TRIANGULAR_MATRIX_MATRIX_BLAS_H

+#include "../../misc/blas.h"
+
 namespace Eigen {

 namespace internal {

[robertodr]

Which version of Eigen is bugged? I can link against BLAS with Eigen 3.3.3 without problems. @stigrj maybe it's time to update Eigen?

[susilehtola]

Mine failed with Eigen 3.3.4 as well as the head branch, with gcc 7.3.1.

[robertodr]

Weird... I'm in favor of pulling this in as-is and then figuring out the issue. @stigrj?

P.S.: The example I was compiling is this one

[stigrj]

Great, -100 lines of code = gold