Better returns for /summary.py functions

[kerbs23]

2 closely related modifications:

1. make the .etable() 'md' mode return the markdown table as a string. Not sure why this isn't done, but also have to sort out the return types of the function.
2. make the .summary() return the summary as a string. requires a bit more work but nothing bonkers

will maybe open a pr with the fixes today or tomorrow when I finish work

[s3alfisc]

We currently print it, correct? I think it's so that it's automatically displayed in a nice format in notebooks.. would users have to wrap it into a print statement in notebooks if returned as a string? If yes I would not be so much in favor.

Btw, se moved almost all code of etable to make tables, so you'd have to implement the changes for etable over there: https://github.com/py-econometrics/maketables