Converting NeuroML2/LEMS to & from SBML

Most of the interactions between SBML and LEMS/NeuroML showcased here are enabled by:

The SBML to LEMS import function in: org.neuroml.importer
The LEMS/NeuroML to SBML export function in: org.neuroml.export

Note these features can be accessed easily with the pyNeuroML tool. For example:

Load LEMSFile.xml using pyNeuroML, and convert it to SBML format:
```
 pynml LEMSFile.xml -sbml
```
Load LEMSFile.xml using pyNeuroML, and convert it to SBML format with a SED-ML specification for the experiment:
```
 pynml LEMSFile.xml -sbml-sedml
```
Load SBMLFile.sbml using jSBML, and convert it to LEMS format using values for duration & dt in ms
```
pynml -sbml-import SBMLFile.sbml duration dt
```

See also https://github.com/ModECI/modelspec/blob/main/examples/COMBINE.md.