Attempt cache thrashing fix for low synapse count vector access #4925

Job	Run time
clang-format	20s
copyright_headers	19s
cppcheck	4m 5s
forbidden_types	12s
rstcheck	19s
unused_names	13s
vale	27s
black	31s
flake8	18s
isort	11s
lychee_IGNORED	21s
mypy	32s
pydocstyle	23s
pylint	31s
sphinx-rtd	3m 1s
sphinx-conda	0s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	2m 7s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	5m 41s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	3m 9s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	2m 58s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	5m 12s
	30m 50s

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