Switch to installing CUTEst via Meson

[jfowkes]

CUTEst and SIFDecode are now installable via Meson. We should switch the CI and documentation over from the old hand-crafted make installer in .install_cutest.sh to the simple meson install approach which requires no updating and is future-proof.

[jfowkes]

See the discussion at ralna/CUTEst#55 for details, the plan for now is to keep the default approach rather than the demuxer as that would be a cache-breaking change best left for a major new release.

[jfowkes]

Also SIFDecoder now has a standalone direct Fortran interface as an alternative to the sifdecode bash script:
ralna/SIFDecode#24
May also be worth switching to this at some point in the future (don't want to break existing installs for now).

[amontoison]

@jfowkes A liitle bit off-topic but I suggest to compile the decoded SIF files with -O3 here.
My CI builds in the Julia interface now require much less time (around 30% less) with this modification.

[jfowkes]

@amontoison good call! I never thought of that, but of course -O3 or -O2 would help a lot with PyCUTEst performance!

[jfowkes]

Also probably worth repurposing the CUTEST, SIFDECODE and MASTSIF environment variables so that they can be used to point to non-default meson installs (i.e. not into /usr/local/) as some users may desire this flexibility.