Integrate Izhikevich spiking cell

[ago109]

• ngc-learn version 1.0.beta0
• Python version: 3.11

Description:
Base Izhikevich cell needs to be integrated into ngc-learn components.

What Was Done:
No errors were thrown -- this feature just needs to be integrated into the core distribution.

[ago109]

IzhikevichCell has been integrated into primary built-in components; under "ngclearn.components.neurons.spiking" (modeling API updated to reflect this change); also previously, FitzhughNagumoCell has been integrated into primary components along with small tutorial demonstrating its use/basic dynamics (as a result, ngc-learn has two 2-ode system cell approximations to the H-H cell)