Document output file structure

[peanutfun]

All operations in this module are output file based, meaning that no function returns its data, but all data is written into output files to be retrieved later on. For this to be viable also in conjunction with other modules, it would be helpful to have the structure of the output documented. Which datasets are written by the, e.g., l96_time_series and filter_state functions, and what are their data types and dimensions? This would particularly help when writing scripts to analyze the results.

The output documentation could be part of the function docstring (see #8), or added to the README.md where appropriate.

openjournals/joss-reviews#4129

[cgrudz]

Examples of function outputs are now included in the doc strings for the various methods of the included submodules, as in e.g., the GenerateTimeSeries submodule:
https://cgrudz.github.io/DataAssimilationBenchmarks.jl/dev/submodules/experiments/GenerateTimeSeries/