Use more specific base classes

[mstimberg]

As we discussed with Padraig, someone using neuron models exported from Brian2 would have it easier if the generated types do not only inherit from the generic baseCell. Compared to the other issues, this is a rather low priority one, but some of the simple classes should be very easy to support, e.g.:

• If the cell defines a threshold, have it inherit from baseSpikingCell
• If it has a threshold and a variable v with dimensionality volt, have it inherit from baseCellMembPot
• If instead it has a threshold and a dimensionless variable V, have it inherit from baseCellMembPotDL

https://github.com/NeuroML/NeuroML2/blob/master/NeuroML2CoreTypes/Cells.xml