Viscosity Momentum Method

Hi all,

It looks like there was a slight error in the momentum equation I derived for the viscosity method. We have assumed previously that tau*dwmax with viscosity is equivalent to dwmax without momentum. I have updated MomentumConn to reflect this assumption more accurately. Luckily, the old equation was only off by a factor of 2; a momentum tau of 100 previously is approximately equal to a momentum tau of 200 for the current iteration. In other words, the old code was decaying more slowly than intended. Formally, the equations changed from

dw = momentumTau * (dwPrev + inpulse) * (1-exp(-1/momentumTau)

to

dw = dwPrev * exp(-1/momentumTau) + inpulse

where

inpulse = dwMax * pre * post .