Add models with exact and simplified NMDA dynamics according to Brunel and Wang (2001) #5417

Job	Run time
clang-format	19s
vale	28s
copyright_headers	17s
cppcheck	3m 36s
forbidden_types	15s
lychee_IGNORED	21s
rstcheck	23s
unused_names	15s
black	32s
flake8	23s
isort	12s
mypy	34s
pydocstyle	18s
pylint	37s
sphinx-rtd	3m 43s
sphinx-conda	0s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	11m 5s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	12m 58s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	18m 19s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	19m 58s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	32m 4s
	1h 46m 37s

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