Create a PLEP on how to implement experimental analysis tools

[namurphy]

One of the most important future capabilities of PlasmaPy will be to analyze experimental data. In many cases, users will need to compare the results to numerical simulations to both interpret the experimental results and validate or improve the numerical simulation. Writing a PLEP to plan development of experimental analysis capabilities will be really helpful, including for writing future proposals.

In our diagnostics subpackage, we have some tools to analyze Langmuir probe data. From the numerical simulation end, we will need mechanisms to create synthetic diagnostics that can be compared against experiments. We also have our base Plasma class thanks to @ritiek and have made some progress on SpectroscoPyx thanks to @lemmatum and @StanczakDominik.

It will be really helpful to have buy-in from plasma experiments. Some challenges will be that most experimental data is not open access and that there are few open standards for storing experimental data.

[pheuer]

@namurphy Might be worth revisiting this idea, and possibly #13 and #17 at the same time?