Simplify JSON format

[pgleeson]

Is your feature request related to a problem? Please describe.

The current specification in json for subelements with multiple children (cells, populations, projections) is a bit excessive in terms of lists/dicts:

       "populations": [
            {
                "pop0": {
                    "size": 5,
                    "component": "iaf",
                    "properties": {
                        "color": "0 .8 0"
                    }
                }
            },
            {
                "pop1": {
                    "size": 10,
                    "component": "iaf",
                    "properties": {
                        "color": "0 0 .8"
                    }
                }
            }
        ],

Describe the solution you'd like
Should be simpler (while still keeping same python API)

        "populations": {
            "pop0": {
                "size": 5,
                "component": "iaf",
                "properties": {
                    "color": "0 .8 0"
                }
            },
            "pop1": {
                "size": 10,
                "component": "iaf",
                "properties": {
                    "color": "0 0 .8"
                }
            }
        }

Fix for this in progress here: https://github.com/NeuroML/NeuroMLlite/tree/update_format

[pgleeson]

Implemented in #12