Add HTML repr for groupby dataframe and series
After I saw the representation of a group by in Airtable, I wanted something similar in pandas. It always bothered me that there was no representation for a group by.

Update: 25.07.2020:
These are old screenshots, but still resemble the look. A new addition is the limitation of groups displayed. I don't know how to make the displaying width across all groups the same. If anyone has an idea, I'd appreciate it.

Update: 26.07.2020:
I've added three tests:

1. All rows in groups and all groups are shown
2. All groups are shown, but not all rows in groups.
3. All rows in groups are shown, but no all groups.

I unsure if it wise to put all cases into one test with an if statement which checks which of the above cases it is. Feedback welcome.

**DataFrame**:
![Screenshot from 2020-06-22 03-13-44](https://user-images.githubusercontent.com/31596773/85239940-75742100-b436-11ea-8b6a-99670d93591a.png)

**Series**:
![Screenshot from 2020-06-22 03-14-46](https://user-images.githubusercontent.com/31596773/85239970-8fadff00-b436-11ea-8f9e-b103c1a3ec2b.png)

This is a WIP implementation. Open issues are:
- [x] improve variable names
- [x] produce more clean HTML code
- [x] limit number of maximum group by DataFrames displayed (similar to the row limit)
- [x] improve look (Did not improve it, but I'm with it)
- [ ] Same width across all groups


- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Truncated groups with a maximum of 3 groups:
![Screenshot from 2020-09-05 16-17-33](https://user-images.githubusercontent.com/31596773/92306995-79becd80-ef93-11ea-86ca-d042ff2116b9.png)
