Create class for recording and plot backpropagating APs, and EPSPs as in examples 3 and 4

We should create functions for recording and plotting backpropagating APs and EPSPs for the ease of the user.

- [ ] See BPAP class in [example 3-bpap](https://github.com/openbraininstitute/BlueCelluLab/blob/main/examples/3-bpap/bpap.ipynb). 

- [ ] We can also create a similar class based on [EPSP example](https://github.com/openbraininstitute/BlueCelluLab/blob/main/examples/4-epsp/epsp.ipynb)