Allow combination of continuous-valued third factor and spike-based third factor

[clinssen]

• Allow combination of continuous-valued third factor and spike-based third factor.
• Allows inline expressions in the postsynaptic neuron model to be used as third factors for the synapse.

TODO: #1258 should be merged into this and the two different buffering methods should be tested.

[github-actions]

Bencher Report

Branch	1264/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 (🔔)	3.91 s (+14.73%) Baseline: 3.40 s	3.74 s (104.30%)

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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 (🔔)	3.91 s (+14.73%) Baseline: 3.40 s	3.74 s (104.30%)

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 (🔔)	3.24 s (-4.67%) Baseline: 3.39 s	3.73 s (86.66%)

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 (🔔)	3.23 s (-4.91%) Baseline: 3.39 s	3.73 s (86.45%)

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