title: how to find the transformation matrix of a change of basis with Eigen
body: <p>I'm porting a program to Eigen.</p>
  
  <p>Now I have to rewrite a method that returns the 3D transformation matrix from one coordinate system A, defined by its origin and two axes, to a second coordinate system, still defined by the origin and two axes.</p>
  
  <p>I was wondering if there's a method in Eigen to find that matrix. I browsed the reference guide but I did not find any useful method yet...</p>
  
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  <p>More in details:</p>
  
  <p>The method I'm porting to Eigen accepts 6 Points (vectors) (fr0, fr1, fr2, to0, to1, to2). "fr0" is the origin of CS1 (Coordinate System 1), "fr1" is a point defining an axis of the CS1, "fr2" is the point defining a second axis of CS1; "to0" is the origin of CS2, and so on... </p>