[eigen] CoeffReturnType causing lots of warning on vs2008

[Kenneth Riddile <kfriddile@xxxxxxxxx>]

I just updated to the latest trunk after being out of the loop for awhile, and I'm getting warnings similar to http://pastebin.com/m1ba91b7e everywhere I instantiate something like:

typedef float Real;

Eigen::Matrix<Real, 4, 4> matrix;