Improved module/package docstrings

[Samuwhale]

This is a first step towards improving the docstrings. Proper docstrings will help the automatically generated API be more usable and informative, it will also help using the package itself.

Next step will be class and function docstrings. But for now, module level docstrings.

I have tried to keep them under 79 characters per line, but in some cases an exception had to be made.

[qubixes]

Thanks a lot! I updated the PR a bit already to remove some linting errors. I really like the new docstrings, more descriptive, without being too verbose! I have added some small comments/suggestions, but nothing major.

[Samuwhale]

I have applied the changes!

I will also create an issue to update the frontpage of the docs to reflect the content of the metasyn __init__.py, and to create an issue to potentially rename MetaVar to MetaSeries.

[qubixes]

Looks good now!