use PointCell params defined in `cellParams` as default values

[vvbragin]

if such params are not customized in popParams

[vvbragin]

need to update the schema

[vvbragin]

The proposal is to avoid the inconsistency we currently have when declaring point cell params in cellParams vs in popParams. E.g., currently the same point cell can be described in cellParams as:

netParams.cellParams['artif_NetStim'] = {
    'cellModel': 'NetStim',
    'params': {'rate': 5}
}

and in popParams as:

netParams.popParams['pop_NetStim'] = {
    'cellModel': 'NetStim', 
    'numCells': 100,
    'rate': 5, 
}

I think it makes sense to use same approach for both cellParams and popParams: either (1) use params dictionary, or (2) have all properties at the top level, without nesting in 'params'.