[eigen] Eigen Needs Forward Declaration Files

[Brad Bell <bradbell@xxxxxxxxxx>]

The standard library file
    # include <iosfwd>
is a forward declarations file for the objects defined in <iostream>.

I have a need of such a file for the object
    class Eigen::SimplicialLDLT;
Here is my reason.

Compiling with eigen and g++ requires one to suppress certain warnings 
as follows:
     -Wno-shadow -Wno-deprecated-declarations
I  wish to limit the amount of source code that these warnings are 
suppressed for.

In order to reduce the amount of source code that requires one to 
include the eigen headers, I am using the ideas in the pimpl idom. To be 
specific, I am using a forward declaration of the SimplicialLDLT class 
in most of my code. Only the code that actually does Eigen calculations 
is including the Eigen headers. For details, see the file
https://github.com/bradbell/cppad_mixed/blob/0b3d3a461c62c052405e31d164b6bb9643b064f1/include/cppad/mixed/cholesky.hpp

There is a problem with this. It seems that the number of template 
parameters in the SimplicialLDLT class changed. To be specific:

In eigen-3.2.1 I find two template parameters
    template<typename _MatrixType, int _UpLo = Lower> class SimplicialLDLT;

In eigen-3.2.5 I find three template parameters
    template<typename _MatrixType, int _UpLo = Lower, typename 
_Ordering = AMDOrdering<typename _MatrixType::Index> > class SimplicialLDLT;

Since eigen has no forward declarations file, this represents a change 
the the Eigen API.