Lowess-grouped

* Make docstrings of lowess_grouped consistent
* Simplify code of lowess_grouped
* Improve testcases:
* Remove unessary code
* Add documentation to each testcase
* Improve readme:
* Polish section "motivation"

* Allow tuples as column names
* Add testcases for the suffix of the smoothed-column
* Improve readme:
* Update introduction image
* Add attribution for the temperature dataset
* Improve example notebook:
* Add example with tuples as column names

- Simplify Source Code
- Improve example notebook:
- Add statments to install the dependencies (e.g. pip install)
- Clarify that the function adds a new column for the smoothed data

- Add testcases
- Fix a bug by updating the supported Python version to 3.8
- The previous version (v0.0.3) already uses 3.8 despite saying it requires 3.6!

- Improve readme:
- Add motivation section
- Add mentions of "loess" and "Savitzky–Golay filter"
- Improve writing
- Make example notebook more concise:
- Explain goal and what we are doing
- Add new headings
- Add docstrings and datatypes to parameters of function lowess_grouped
- Adjust requirements.txt to consider newer versions

- Improve readme:
- Add example image
- Make usage section more concise, e.g. by adding pip installation
- Add reference to statsmodels package