FixedArray internal struct was not respecting Eigen alignment.

Added EIGEN_MAKE_ALIGNED_OPERATOR_NEW to the struct so that
the Jet members are aligned. This fixes an eigen assert in
autodiff_test reported by multiple Ceres users.

Thanks Koichi Akabe & Stephan Kassemeyer

Change-Id: Id3574e926deffa57d205dddaa9d08389b5dc33a8