Reactivate Min-max processing code added to pycomlink with API from v0.2.x

[cchwala]

Code added in #37 and bugfixed in #45 was droppend when we switched to using xarray.Dataset as internal data structure starting in version 0.3.0.

It would be good to bring back this code and integrate it into the new internal structure. Only the two functions introduced in #37 will be required. I do not remember the details how they are used. But this should be clear from the example notebook used back then.