MNT: Work-around `armadillo` performance regression

MMORF v0.4.0 was found to have substantially slower runtime than v0.3.3 for a couple of reasons:

• A change in newimage which has since been resolved (no changes needed to MMORF)
• A subtle performance regression in armadillo

The armadillo issue has been resolved upstream, but as we will keep using armadillo 14.2.1 for some time, a work-around is needed in the MMORF code. When we upgrade armadillo this work-around can be removed, although it is not particularly burdensome.

This MR also makes a few minor changes to the MMORF Makefile to work with the latest version of the FSL build system.