Improve documentation

of all user-facing functions. They don't have to be perfect the first time but they should be at least usable.

• ec
• ecxsys
• plot_ecxsys
• Stressconversion

Look at indicate for inspiration.

Examples for ecxsys:

• Example use of effect_max. Just make effect_max 1 and divide all input by 100.
• Example for how to use fn() to generate the values at specific concentrations.