API: Change add_tonic_bias argument structure

[gtdang]

Update the Network add_tonic_bias method to accept a dictionary of cell types and amplitudes. As discussed in #635.

[ntolley]

Adding on to this issue, we should add a name parameter to this function: net.add_tonic_bias(..., name='tonic1')

It's not unreasonable to add multiple tonic biases to the same cell. Many electrophysiology experiments have multiple stages to the "biases" added, for example hyperpolarizing all of the cells at the beginning, and then depolarizing immediately after.

[jasmainak]

@ntolley I think net.biases is a list rather than dictionary? How would we store the name?

[gtdang]

I think it's currently set like below. I believe it's a multi-level dict.

self.external_biases['tonic'][cell_type] = {
            'amplitude': amplitude,
            't0': t0,
            'tstop': tstop
        }

Would it make the most sense to add a layer between "tonic" and "cell type"? self.external_biases['tonic'][<NAME>][cell_type].

Or could the default name 'tonic' just be modified?

Edit: @ntolley and I discussed and name parameter would specify the key of the top level dictionary as follows. This would be in a separate PR than #764.

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