Check if model attribute with name given by group_name corresponds to a list before adding to that list

[rgerkin]

52ffdca:
Some models have attributes that are not lists, resulting in an error when we try to append to that list. This commit converts the attribute to a list, if needed, before adding to it. Here is an example of such a model, where the ChannelPopulation attribute was not a list, but ended up getting passed here. I assume the original problem is upstream in libNeuroML, but I'm not sure where, and this commit is sufficient for getting that model to run in pylems.

ed435e2:
I want to use pylems from inside my other scripts, and calling the pylems shell script is awkward, so I added the ability to run pylems directly via python by wrapping the main() function and creating a dummy argument parser filled from arguments to a new run() function.

[pgleeson]

Thanks @rgerkin, a number of models that were failing in pylems are working now. Checking some of your other issues now...