title: How to create an efficient 3D matrix using the Eigen library in C++?
body: <p>For a robotics project I need a 3D matrix of a pre-defined size. Using the Eigen library, I don't see how: 
  1.) to create a large pre-defined matrix, seems like I have to use matrixXd but that's for a dynamic matrix. 
  2.) create a 3D matrix, i. e. size = (int from 200 to 1000) and <code>matrix (size, size, size)</code></p>