API: Add name parameter to add_tonic_bias method

[gtdang]

Discussed in #764 Add a name parameter to this method: net.add_tonic_bias(..., name='tonic1'). The name will add a external bias key of the name supplied:

self.external_biases[<NAME>][cell_type] = {
            'amplitude': amplitude,
            't0': t0,
            'tstop': tstop

Currently only one external bias with the default name 'tonic' could be added to the network. Areas where this is key is referenced would need to be updated to handle multiple external biases.