Add __version__ attribute

[mehmetcanay]

• Added _version.py to hold version value in __version__
• Changed the publish action to substitute the version in _version.py instead of setup.py (the regex needs to be checked).
• Imported __version__ from _version.py to __init__.py to expose the attribute. The user should be able to print the version of the package with the command datastew.__version__.

closes #38

[tiadams]

Can you add a unit test that simply checks if datastew.__version__ is in the fornat x.y.z?

[mehmetcanay]

Can you add a unit test that simply checks if datastew.__version__ is in the fornat x.y.z?

I can also extend this unique test if you'd like to check if the version retrieved is in the canonical format, if in the future you would like to use prefixes like a, b, rc. PEP already provides a function for it in https://packaging.python.org/en/latest/specifications/version-specifiers/#appendix-parsing-version-strings-with-regular-expressions