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 unittest
Rewrite tests using nose. Previous version was not really working.

The problem with the previous version of the test file was that
unittest was not considering the validity of each LEMS definition file
and each NeuroML example as a separate test, so it was overall pretty
useless.