Merge branch 'development' of github.com:NeuroML/NeuroML2 into development
Adding ability to add <property tag=".." value=".."/> to populations (not only inside <annotations>))

Will be added for other elements too.
Adding test on ex6 (nmda-r synapse)
Allowing multiple different combinations of gateHHtauInf/gateHHratesInf in ionChannel
Merge branch 'development' into experimental

* development:
  Allowing multiple different combinations of gateHHtauInf/gateHHratesInf in ionChannel
  Adding test on ex6 (nmda-r synapse)
  Adding ability to add <property tag=".." value=".."/> to populations (not only inside <annotations>))