add armadillo 15.2.x

[conradsnicta]

@RUrlus Update default to Armadillo 15.2.x.

Changes in Armadillo 15.2 since 15.0:

• workarounds for bugs in GCC and Clang sanitisers (ASAN false positives)
• faster .resize() for vectors
• faster repcube()
• faster handling of submatrices with one row
• faster handling of blank sparse matrices
• improved reproducibility of random number generation when using OpenMP
• added rande() for generating matrices with elements from exponential distributions
• shift() has been deprecated in favour of circshift(), for consistency with Matlab/Octave
• reworked detection of aliasing, leading to more efficient compiled code

[RUrlus]

@conradsnicta thanks again! Apologies for the delay, wanted to get the CICD pipeline in order but I'll have to leave that for another time