Support SpikeSourceCells

[thorstenhater]

Example

    <SpikeSourcePoisson id="poisson_input" start="0.0ms" duration="1000000000.0ms" rate="50.0Hz"/>

found in the wild in

https://github.com/OpenSourceBrain/GranCellLayer

under MaexDeSchutter1998.net.nml.

Should be easy enough, just add as Arbor's spike_source_cell, but the logic to sort out gid -> cell_kind
becomes less trivial.