Allow detection of merged synapse core vs. split cores at any time step

[rowleya]

in #1558, split synapse cores will only be automatically detected to be needed when the time step is less than 1.0ms. It should be possible to know if you can do the synapses on a merged core with neurons vs. on a separate core regardless of timesteps. This would need a bit more information in terms of approximately how much CPU time neurons take.