Refactor the code from Manuel Rexer

Overall refactoring of the code and creation of tests that also could be used to validate other implementations.

@manuel.rexer Please give overall feedback and I'm unsure if I described the 'propagateSysUncToFreqDomain(...)' and 'calculateSysUncInFreqDomain()' correctly. So please have a closer look at the docstrings of those two functions and correct me if I have described something wrong. Also please check the README.md if you are happy with the structure and the content and if something important is missing.
Thanks.

I didn't implement the test of the data structure since It can be controlled by hand easily. But If there should be advances in the use of RDF in calculation packages/software we should come back to it and implement it.

Open TODOs:

1. The main function is quite short. Should I still leave in the short code example on how to use it? I think a real world example with a plot of a data signal would be quite cool.
2. Should I add a short documentation of the output variable 'result_struct' that the user knows what it contains and how to use it? If yes, I'm going to use a picture with some kind of directory-tree structure
3. The repository still needs a license

If you are happy with the current state you can do the merge-request by yourself.
I will open a new one for the documentation and the license.