[eigen] Using Eigen with CppAD scalar types

[Brad Bell <bradbell@xxxxxxxxxx>]

I think that I have correctly connected CppAD::AD<Base> types to Eigen 
(for the case where Base is a real type).

Here is the Eigen custom scalar type interface
     http://www.coin-or.org/CppAD/Doc/eigen_det.cpp.xml

Here is an example usage
    http://www.coin-or.org/CppAD/Doc/eigen_det.cpp.xml

Here is the corresponding C++ source code
    
https://projects.coin-or.org/CppAD/browser/trunk/cppad/example/cppad_eigen.hpp
    https://projects.coin-or.org/CppAD/browser/trunk/example/eigen_det.cpp