Update eigenvalues() and operatorNorm() methods in MatrixBase.

* use SelfAdjointView instead of Eigen2's SelfAdjoint flag.
* add tests and documentation.
* allow eigenvalues() for non-selfadjoint matrices.
* they no longer depend only on SelfAdjointEigenSolver, so move them to
  a separate file

doc/snippets/MatrixBase_eigenvalues.cpp

@@ -0,0 +1,3 @@
+MatrixXd ones = MatrixXd::Ones(3,3);
+VectorXcd eivals = ones.eigenvalues();
+cout << "The eigenvalues of the 3x3 matrix of ones are:" << endl << eivals << endl;