Update Armadillo to 4.000.0

Release summary from http://arma.sourceforge.net
	
Version 4.000   (Feral Steamroller)
- added eigen decompositions of sparse matrices: eigs_sym() and eigs_gen()
- added eigen decomposition for pair of matrices: eig_pair()
- added simpler forms of eig_gen()
- added condition number of matrices: cond()
- expanded find() to handle cubes
- expanded subcube views to access elements specified in a vector
- template argument for running_stat_vec expanded to accept vector types more robust fast inverse of 4x4 matrices
- faster divide-and-conquer decompositions are now used by default for eig_sym(), pinv(), princomp(), rank(), svd(), svd_econ()
- the form inv(sympd(X)) no longer assumes that X is positive definite; use inv_sympd() instead
- added MEX connector for interfacing Octave/Matlab with Armadillo matrices (contributed by George Yammine)