Split requirements

[marimeireles]

Currently the requirements file contains packages relevant to building up the docs, but not all necessary packages to installing whynot.
I propose using the setup.py file to separate the different necessary packages:

    install_requires=[

    ],
    extras_require={
        "test": [
 
        ],
        "examples": [

        ],
        "docs": [
        ],
    }

That will allow us to:

1. delete the requirements.txt file as it will be no longer needed
2. users will now be able to choose the different pkgs they wish to install
   pip install -e . will install whynot alone.
   pip install -e ".[test, docs]". would install both whynot and the pkgs necessary for test and docs, for example.