Import Eigen3 v.3.2.1 (sans MSVC debug support)

The software was released on the 26th of February 2014.  See

http://eigen.tuxfamily.org/index.php?title=ChangeLog#Eigen_3.2.0
http://eigen.tuxfamily.org/index.php?title=ChangeLog#Eigen_3.2.1

for a high-level overview of the changes in versions 3.2.0 and 3.2.1.

Notably, this release series brings support for manipulating sub-matrix blocks of objects of class `Eigen::SparseMatrix<>`.