refactored into a python package

[thawn]

created pyproject.toml and refactored the code into an installable python package.

the code can be installed like this:

git clone ...
cd Time-Series-Library
pip install -e .

afterwards the code can be used like this:

from timeserieslib.models import TimesNet

[thawn]

closes #41

[wuhaixu2016]

Hi, this is a great pull request. It may take some time for us to merge and reorganize the repo. Many thanks for your contribution.

[thawn]

I am aware that this is a large refactor and will take time to review. Please let me know in case I can help (e.g. rebase).