GPU todo (later)

[rcoreilly]

There are various things that could be done on the GPU, which are currently only on CPU, but are low-cost and thus low-pri:

• PlusPhasePost (matrix gating, corsim stat, act avg updates)
• HebbPrjn type -- needed for lateral inhib and hippocampus
• DWt is using Context.NeuroMod for all DA, ACh values -- in principle should use LayerVals.NeuroMod in case a layer does something different. can fix later as needed.