Matrix learning rule: should sub-pools only get credit if they were the ones that drove Gating

[rcoreilly]

Previously, we only learned for subPools that got above the gating threshold:

      prjnparams.go:369:
	if subPool.Gated.IsTrue() {
		dtr = rn.SpkMax * sn.CaSpkD // we will get the credit later at time of US
	}

However, this does not really make sense for the NoGo case where there is not a strong pool-level logic..

here's the comment:

	// let's not worry about giving credit only to the sub-pool for now
	// if we need to do this later, we can add a different factor for
	// D2 (NoGo) vs D1 (Go) -- the NoGo case should *not* care about subpools
	// in any case.  Probably the Go case can not care too.

this ticket is just a reminder to remember this and revisit should some kind of parasitic activity develop in the Go layers as a result!