add armadillo 12.8.x

[conradsnicta]

@RUrlus Add armadillo 12.8.x branch and use it as default

Changes since Armadillo 12.6.x:

• workaround in norm() for yet another bug in macOS accelerate framework
• faster detection of symmetric expressions by pinv() and rank()
• expanded shift() to handle sparse matrices
• expanded conv_to for more flexible conversions between sparse and dense matrices
• added cbrt()
• more compact representation of integers when saving matrices in CSV format

[RUrlus]

Thanks @conradsnicta, I'll fix the failed tests and merge when done.