doc-string doc of test functionality

Documentation of testing functionality has been moved from repo located markdown files to in-code doc strings.

@schaefed, @palmb, @gellerj - you might want to check the plausibillity of documentations of functions you implemented.

@schmidle - i didnt format/document the sm_random forest method, cause i am not sure about its status. You may want to add its documentation to the merge request.

I am not sure where to put detailed mathematical descriptions of functions, that may not be well explained in text only. For now, i added a markdown file to the repo:

https://git.ufz.de/rdm-software/saqc/-/blob/testfuncDocs/docs/funcs/FormalDescriptions.md

I guess we will have short talk about how and where to store additional docu content.

Some formatting remarks:

• I slightly deviated from the how-to-doc protocol.
• I didnt use the Union type when denoting different parameter data types. I used curly brackets.
• I didnt refer to optional parameters in the numpy style, but in the pandas style.

In general, the template i used is exemplified by:

"""
paraFoo : {None, str}, default None
    ParaFoo is such an important parameter. Never forget to set it.
paraBa : Callable[float, float], default np.log
    paraBa is not so important.
""" 

I found it to be well balanced between precision and uglyness...