Make recording a mechanism

[michaeldeistler]

Update API to insert recordings. The new way is closer to how NEURON does it and it allows removing some boilerplate code.

Old API

recs = [nx.Recording(cell_ind, 0, 0.0) for cell_ind in range(num_cells)]
s = nx.integrate(network, stimuli=stims, recordings=recs, delta_t=dt)

New API

for cell_ind in range(num_cells):
    network.cell(cell_ind).branch(0).comp(0.0).record()

s = nx.integrate(network, stimuli=stims, delta_t=dt)

[michaeldeistler]

@coschroeder FYI