Add multi-state neuron models #6343

Job	Run time
clang-format	21s
cppcheck	8m 59s
vale	37s
rstcheck	25s
copyright_headers	20s
unused_names	19s
lychee_IGNORED	26s
forbidden_types	18s
pylint	46s
black	31s
flake8	30s
pydocstyle	25s
isort	17s
mypy	36s
sphinx-rtd	3m 26s
sphinx-conda	0s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	10m 34s
build_linux (ubuntu-22.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	20m 41s
build_linux (ubuntu-22.04, gcc, boost, optimize, warning)	12m 16s
build_linux (ubuntu-22.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	34m 4s
build_linux (ubuntu-22.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	19m 9s
	1h 55m 0s

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