add armadillo 12.6.x

[conradsnicta]

@RUrlus Add armadillo 12.6.x branch and use it as default

Changes since Armadillo 12.4.x:

• added diags() and spdiags() for generating band matrices from set of vectors
• faster multiplication of dense vectors by sparse matrices (and vice versa)
• faster eigs_sym(), eigs_gen(), svds()
• faster conv() and conv2() when using OpenMP
• use thread-safe Mersenne Twister as the default RNG on all platforms
• use unique RNG seed for each thread within multi-threaded execution (such as OpenMP)

[RUrlus]

Thanks @conradsnicta