Test round trip SBML->LEMS-> SBML

[pgleeson]

No description provided.

[pgleeson]

Main issue is that the assumption in the LEMS/NML2->SBML converter is that the nml model is structured as a network of populations. The converter creates an sbml compartment for each element of a population.

However the SBML->LEMS converter just produces a simple component at top level in the lems which is the target of the simulation.

Solution: support the single top levlel componenbt option in this: https://github.com/NeuroML/org.neuroml.export/blob/development/src/main/java/org/neuroml/export/sbml/SBMLWriter.java#L89

@vporubsky