Replaced Gomill with its submodule

[ghost]

The Gomill project is far ahead of what is present in GRP. And there are already two commits in GRP doubling the work in the original project further in its history. I thought it might be a good idea to keep Gomill as a submodule to easily update it in GRP. The size difference is only 1M.

For a release, the README.txt with licence has to be moved or copied from gomill/gomill_setup to gomill/gomill and all the rest can be removed including the dot files.

Submodules introduce a bit of turbulence into main repositories, so I advise to test in a separate clone.

A documentation update would have to be applied.

[pnprog]

Hi,

Thanks for the PR, and sorry for low reactivity, I slowly trying to get back into GRP development.

I had never heard about "submodule" until today, so I will need a bit of time to learn about that before validating your commit. GRP also uses two other projects that could be moved to submodules as well.

[Nihisil]

Why not using python package instead of git submodule? https://pypi.org/project/gomill/