Check error when plotting blocking traces in NMDA example

[pgleeson]

See: https://github.com/NeuroML/NML2_LEMS_Examples/blob/master/Figure5_NMDA.xml#L111

When this commented out code is added back (and population rampPop is readded), model works in jLEMS:

but throws error in pylems:

Traceback (most recent call last):
  File "/home/padraig/pylems/pylems", line 13, in <module>
    main()
  File "/home/padraig/pylems/lems/run.py", line 96, in main
    sim = SimulationBuilder(resolved_model).build()
  File "/home/padraig/pylems/lems/sim/build.py", line 63, in build
    runnable = self.build_runnable(component)
  File "/home/padraig/pylems/lems/sim/build.py", line 179, in build_runnable
    self.process_simulation_specs(component, runnable, component.simulation)
  File "/home/padraig/pylems/lems/sim/build.py", line 627, in process_simulation_specs
    target = self.build_runnable(run.component, runnable, cid)
  File "/home/padraig/pylems/lems/sim/build.py", line 182, in build_runnable
    child_runnable = self.build_runnable(child, runnable)
  File "/home/padraig/pylems/lems/sim/build.py", line 199, in build_runnable
    self.build_structure(component, runnable, component.structure)
  File "/home/padraig/pylems/lems/sim/build.py", line 312, in build_structure
    runnable)
  File "/home/padraig/pylems/lems/sim/build.py", line 191, in build_runnable
    runnable.add_child_to_group(children.name, child_runnable)
  File "/home/padraig/pylems/lems/sim/runnable.py", line 186, in add_child_to_group
    self.__dict__[group_name].append(child)
AttributeError: 'Runnable' object has no attribute 'append'