Add examples of fractional neuron models

[alexfikl]

Mostly to showcase the library and to reproduce some results from the literature. These should be as simple as picking a discretization and re-running an example from a paper.

• Integrate-and-Fire: PIF
  • Would need a better example to compare to?
• Integrate-and-Fire: LIF
  • Would need a better example to compare to?
• Integrate-and-Fire: EIF
• Integrate-and-Fire: AdEx
• FitzHugh-Nagumo: https://doi.org/10.1002/mma.4768
• FitzHugh-Rinzel: https://doi.org/10.1038/s41598-019-52061-4
• Morris-Lecar: https://doi.org/10.1016/j.cnsns.2013.10.032
• Izhikevich: https://doi.org/10.1016/j.cnsns.2017.07.026
• Hindmarsh-Rose: https://doi.org/10.1007/s11571-013-9273-x
• Extended Hindmarsh-Rose: https://doi.org/10.1016/j.chaos.2018.11.028
• Hodgkin-Huxley: https://doi.org/10.1371/journal.pone.0126629