Add cable cell option to Brunel example

[thorstenhater]

1. Add the option to use a simple HH cell to the Brunel example.
2. Add correct group size controls to Brunel
3. Add more integration tests
4. Make lif_cell an aggregate, which enables C++20 designated initializers i.e. lif_cell {.Vm=42_mV}.

This enables us to

• test/benchmark the event delivery in the cable cell path (Brunel is quite event heavy)
• add a few integration tests checking the invariance across CPU/GPU and group size with both paths
• add optimizations of said code paths later.

[thorstenhater]

Note that there are some tests failing. These occur consistently when building with the Apple Clang Compiler Suite.
Potentially this is an instance of:
https://stackoverflow.com/questions/76407947/trivial-c-program-yields-different-result-in-clang-macos-arm64-and-clang-macos-x
I'll still merge, since nothing functional changed here, but spin off an issue at high priority to investigate.