Implement e-prop plasticity #4655

Job	Run time
copyright_headers	25s
clang-format	17s
vale	29s
cppcheck	4m 6s
forbidden_types	13s
lychee_IGNORED	26s
isort	11s
mypy	32s
pydocstyle	29s
pylint	41s
rstcheck	26s
unused_names	13s
black	32s
flake8	29s
sphinx-rtd	8m 57s
sphinx-conda	0s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	6m 31s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	7m 28s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	12m 38s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	7m 56s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	10m 17s
	1h 3m 16s

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