Added PyNN standard cell/synapse/input types (IF_curr_alpha, etc) to Schema & now example NML2_PyNNCells is valid.
Tidied up (comments etc) some of the standard NML2 examples
Removed internal current from AdExCell (attributes Iamp/Idur etc.) as external pulse generator is better way to apply curent, see LEMS_NML2_Ex8
Added gateHHratesInf to Channels.xml, as this is needed if alpha/beta & inf are specified in channel
Added Hz as standard unit
Merge branch 'master' into stp

Conflicts:
	Schemas/NeuroML2/NeuroML_v2beta.xsd
Fix typo in plasticSynapse definition.
Update STP example (Ex7) to use plasticSynapse instead of stpSynapse.