Document new logging options

[NikoOinonen]

The new logging system was merged in #368, this needs to be documented in the wiki for the CLI and readthedocs for the Python API.

[NikoOinonen]

I added a section to the wiki here: https://github.com/Probe-Particle/ppafm/wiki/Command-line-interface#controlling-logging-level

@ondrejkrejci @mondracek Could at least one of you take a look if it makes sense.

[ondrejkrejci]

Hi, I went through - it makes sense to me. The only information missing there is what happens if the log file is not set.

[NikoOinonen]

The only information missing there is what happens if the log file is not set.

I changed it to read "By default (when not set) prints to terminal stdout.". Would that be more clear?