Add option for display/editing of cell list

Specifying a list of cell positions is a standard feature of NetPyNE populations, and will be crucial for loading/displaying NeuroML models, where the positions are generated in NeuroML (e.g. with pyNeuroML or NeuroMLlite) and exported to NetPyNE using "cellsList" in the popParams element, e.g. https://github.com/OpenSourceBrain/NetPyNEShowcase/blob/0fa6c7032d9b2984f169d48773e573e16b9a8505/NetPyNE/UI/HHCellNetwork.txt.json#L99.

However this is not curently supported by the UI. The cellList etc. is probably loaded in the background by netpyne core, and possibly used for generation (although possibly not, see https://github.com/MetaCell/NetPyNE-UI/issues/213), but the user can't see this. Adding this feature in the UI will make NML imported models (and an other models that use cellList) more transparent & reproducible.

Start by tracing how NetPyNE-UI loads and edits population parameters, especially the cellsList field in popParams. Review issue #213 and the linked NetPyNEShowcase example to understand whether the list is only loaded or also used during generation. Done means users can see and edit cell positions in the UI and the values remain transparent and reproducible for imported NeuroML models.

Written by the indexing model from the issue text.