move analysis code from HNN GUI to start hnn_core.analysis submodule

[blakecaldwell]

Purpose:
To make code that was previously only in HNN GUI dual-purpose and serve for the analysis of simulations run by hnn-core from the command-line.

Rationale:
Since HNN GUI will import hnn-core already for hnn_core.simulator to run the NEURON solver, this will be a better home for the analysis functions. These functions will benefit from documentation and unit testing as part of hnn-core

Proposed components:

• Simulated LFP in hnn_core.simulator (WIP: Add ability to record LFP's #150)
• CSD calculation and plotting (add CSD plot for comparing against animal studies #68)
• Objective functions for model optimization (parts from [MRG] Optimization example script #77)
• Wrappers for parameter estimation with SNPE
• Spectral analysis
• RMSE calculation to data (RMSE and dipole averaging #83 - closed, needs PR)

In addition to moving the code, the following improvments will be made:

• Code cleanup (Use library functions, formatting, etc.)
• Unit testing
• Documentation

[blakecaldwell]

It's not clear whether optimization code should go entirely in analysis, or the wrapper that runs code in hnn_core.simulator should also go in hnn_core.simulator.

[jasmainak]

@blakecaldwell can you cross-link to already open pull requests and issues? For e.g., for rmse we already have a pull request and also for LFP/CSD.

[ntolley]

Going through the list all of these have actually been implemented in some capacity!