refactor: excel to json list (DEV-431) (#155)

* Don't open the file each time in the recursive function get_values_from_excel()

* instead of side effects, all methods should return the result they computed

* - make code more understandable by improving the logic
 - improve docstrings and annotations

* improve docstrings and annotations

* - various style/annotation improvements
 - remove side-effect of get_values_from_excel by making a copy of parentnode before returning it

* - correct mistake in logic
 - improve error message output

* - reconstruct the side-effect from before

* strip strings

* adapt 'expand_all_lists.py' to the refactorings

* elminate little bug

* introduce the side effect again

* rename variable

Co-authored-by: irinaschubert <irina.schubert@dasch.swiss>

* correct previous error

* improvements suggested by reviewer

Co-authored-by: irinaschubert <irina.schubert@dasch.swiss>

* implement reviewer's feedback

Co-authored-by: irinaschubert <irina.schubert@dasch.swiss>

Author: jnussbaum