Regenerated for latest version of NeuroML 2 schema

Added ion_channel to the NameTable to ensure correct naming of attribute
of ChannelDensity:

ChannelDensity(cond_density='50 mS_per_cm2', ion_channel='NaF',
erev='55.0 mV', ion='na')

This has the unfortunate consequence that the array in doc is singular:
doc.ion_channel.append(chan)
See #46