Check if test on [Ca2+]->0 is needed in SK_E2

See original mod:
https://github.com/OpenSourceBrain/L5bPyrCellHayEtAl2011/blob/master/neuroConstruct/cellMechanisms/SK_E2_mod/SK_E2.mod

Condition if(ca < 1e-7) is not checked in ChannelML.
