Missing documentation

[dlohmeier]

I would like to summarize some other issued and make a collection of what's still missing in the documentation. I found:

• set_user_pf_options: some better documentation (cf. Documentation of set_user_pf_options #425 ), please improve on https://pandapipes.readthedocs.io/en/latest/pipeflow/options.html#user-options
• press_control component (cf. Documentation pressure_control missing #414 )
• flow_control component
• compressor component
• update hard-coded parts for the new components (e.g. for plotting, cf. https://pandapipes.readthedocs.io/en/latest/plotting/simple_plot.html)
• possibly some parts of time series (cf. Time Series Documentation #224 )
• toolbox functions (some were added, some might need updates)
• ways to run a heat transfer simulation with the mode to choose and coupling to hydraulics simulations (cf. Meaning of the `hyd_flag` parameter in pipeflow options #424 )
• explicitly state that the Nikuradse friction factor is the sum of the laminar and turbulent lambda (to get a smooth function)
• add a physical model description for new components (controllers, compressor, heat exchanger)
• in function create_pipe (in file create.py), should the doc string not say "Creates a pipe element [..] from a pipe standard type"? If yes, correct it (pending in PR (tuto) Create network out of csv files and using "plural" create functions #519).
• Restructuring of the fluid chapter (explain how fluids work, then give insights to the current library, then some toolbox) + add some more description

If you find any further parts that are missing or need to be updated, please extend the list.