Add SpikeGenerator with delay, parameterized noise

[sanjayankur31]

• one with delay where the start time can be specified separately from the period (SpikeGenerator has a period)
• another that allows setting noise to follow NEURON's netstim: https://nrn.readthedocs.io/en/8.2.2/python/modelspec/programmatic/mechanisms/mech.html#NetStim