Adding type gateHHratesTauInf, for when alpha, beta, tau & inf are all specified (& tau depends on alpha/beta but not inf)

e.g. see here:
https://github.com/OpenSourceBrain/L5bPyrCellHayEtAl2011/blob/master/neuroConstruct/generatedNeuroML2/Nap_Et2.channel.nml
This was generated from:
https://github.com/OpenSourceBrain/L5bPyrCellHayEtAl2011/blob/master/neuroConstruct/cellMechanisms/Nap_Et2/NaChannel_HH.xml