ensure compatibility with NESTML custom as well as NEST built-in syna… #1081

Job	Run time
static_checks	48s
link_checker	15s
build_and_test_py_standalone	1m 53s
build_and_test_nest_compartmental (master)	15m 12s
build_and_test_nest (v2.20.2)	28m 40s
build_and_test_nest (v3.0)	30m 9s
build_and_test_nest (v3.6)	28m 57s
build_and_test_nest (master)	1h 0m 51s
	2h 46m 45s

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