Fix bug when strictly_synaptic_vars are specified as code generator options

[clinssen]

@pnbabu: you already caught and fixed some bugs related to this code generation option by changing "append" to "extend"; this PR makes the same kind of fix.

I also added in some text from the NESTML paper and added a reference to the strictly_synaptic_vars code generation option.

[github-actions]

Bencher Report

Branch	1246/merge
Testbed	ubuntu-latest

🚨 1 Alert

Iteration	Benchmark	Measure Units	View	Benchmark Result (Result Δ%)	Upper Boundary (Limit %)
0	tests/nest_continuous_benchmarking/test_nest_continuous_benchmarking.py::TestNESTContinuousBenchmarking::test_stdp_nn_synapse	Latency seconds (s)	📈 plot 🚷 threshold 🚨 alert (🔔)	3.79 s (+10.38%) Baseline: 3.43 s	3.78 s (100.35%)

Click to view all benchmark results

Benchmark	Latency	Benchmark Result seconds (s) (Result Δ%)	Upper Boundary seconds (s) (Limit %)
tests/nest_continuous_benchmarking/test_nest_continuous_benchmarking.py::TestNESTContinuousBenchmarking::test_stdp_nn_synapse	📈 view plot 🚷 view threshold 🚨 view alert (🔔)	3.79 s (+10.38%) Baseline: 3.43 s	3.78 s (100.35%)

Benchmark	Latency	Benchmark Result seconds (s) (Result Δ%)	Upper Boundary seconds (s) (Limit %)
tests/nest_continuous_benchmarking/test_nest_continuous_benchmarking.py::TestNESTContinuousBenchmarking::test_stdp_nn_synapse	📈 view plot 🚷 view threshold 🚨 view alert (🔔)	3.17 s (-7.36%) Baseline: 3.42 s	3.76 s (84.22%)

Benchmark	Latency	Benchmark Result seconds (s) (Result Δ%)	Upper Boundary seconds (s) (Limit %)
tests/nest_continuous_benchmarking/test_nest_continuous_benchmarking.py::TestNESTContinuousBenchmarking::test_stdp_nn_synapse	📈 view plot 🚷 view threshold 🚨 view alert (🔔)	3.20 s (-6.53%) Baseline: 3.42 s	3.76 s (84.97%)

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