ACh Thr, Range params, layer-level vals

[rcoreilly]

Add Thr, Range params for layer-level ACh effects, so each layer has its own sensitivity dynamics to the global ACh levels

This is needed for managing more graded levels of ACh as discussed in #281

The prjnparams.go DWtSynBLA method already has this baked-in threshold:

} else if GlbV(ctx, di, GvACh) > 0.1 {

that's not good...

This then feeds into AChDisinhib and ACh lrate modulation etc.