CoreNEURON cell grouping

[halfflat]

In the current instantiation of CoreNEURON support, benchmarks are run with a suboptimal grouping of cells for GPU execution. This is partly due to the sharing of model set up across NEURON and CoreNEURON.

The performance can be addressed by making one large group of cells for CoreNEURON. This may require more fundamental changes to the way the benchmark input data is generated for CoreNEURON.