Update armadillo to the supported stable release.

Changes since version (5.600)

• correction for datum::Z_0 constant
• fixes for corner cases in gmm_diag class
• fixes for spsolve(), eigs_sym(), eigs_gen(), svds()
• advanced constructors for using auxiliary memory now have the default of strict = false
• Cube class now delays allocation of .slice() related structures until needed
• stricter handling of matrix objects by hist() and histc()
• added conv2() for 2D convolution
• added stand-alone kmeans() function for clustering data
• added trunc()
• added ind2sub() and sub2ind()
• added .for_each() to Mat, Row, Col, Cube and field classes
• added rcond() for estimating the reciprocal condition number
• added Schur decomposition: schur()
• expanded solve() to find approximate solutions for rank-deficient systems
• expanded join_slices() to handle joining cubes with matrices* faster handling of non-contiguous submatrix views in compound expressions
• expanded each_col(), each_row() and each_slice() to handle C++11 lambda functions
• expanded diagmat() to handle non-square matrices and arbitrary diagonals
• expanded trace() to handle non-square matrices
• extended conv() to optionally provide central convolution
• faster handling of multiply-and-accumulate by accu() when using Intel MKL, ATLAS or OpenBLAS
• faster norm() and normalise() when using Intel MKL, ATLAS or OpenBLAS