Add a mechanism to allow spectrum of time constants in 6CT neurons

[rcoreilly]

Available neurophys evidence suggests that 6CT neurons show a spectrum of different temporal delays, and in general such temporal spectral coding models are efficient at covering a range of different temporal contingencies -- essentially just coarse coding in the time domain.

Temporal integration beyond immediate last "trial" depends on self cons. Weights there can influence some amount of individual diff in sensitivity, but main factor is Rel prjn scaling which downscales sensitivity to other inputs while upscaling self -- that would need to be settable on a per neuron basis.. also investigate whether simple self one-to-one vs. broader integration is better?