[issue-307] implement validation

[armintaenzertng]

will fix #307

[meretp]

Thanks for all the effort and especially dealing with the whole spec to get all validations, that must have been quite a lot! I have not dealt with the spec in such detail, but I think that you managed to implement all possible validations (except for the open issues of course), congrats on that! 🥇
I left quite a lot comments but most of them deal with double quotes or unused imports. I also left a comment about the architecture what we possibly should discuss. I have to admit: The more I looked into your code, the more unsure i became whether your way isn't the most suitable after all.

[meretp]

Looks very good to me, thanks for addressing the other comments! I have only a few small remarks left.
One remark concerning usage: To test the validation I used the following code

test_pack = Package(spdx_id="spdxid", name="jusst a test", download_location="I have to put this here")
validate_package(test_pack)

and got an error Missing context. As ValidationContext can be instantiated without further parameter, I was wondering if we can make this parameter optional for all validation-methods. What do you think?

Edit: We should also add a license header to the new files.

[meretp]

Looks good to me, thank you very much for addressing all comments! 👍