Add License to all of `src/adiar`

[SSoelvsten]

We should include a license statement in each of the files regardless of the license in question.

Where to Include the License

Public files

Include the license in the way one usually does it: at the top.

• [ ] *.h files in <adiar/*>

Internal files

Based on this, let us include the license information in a slightly easier way. Here, to make life easy while developing, let us only include a short copyright notice at the top like the one below.

// MIT License
// -----------
//   Copyright (c) 2020 : Steffan Christ Sølvsten
//   See bottom of this file for the entire MIT license.

Then the entire MIT license is included at the bottom.

• [ ] *.h files in <adiar/internal/**>
• [ ] *.cpp files in <adiar/**>

Who to Include in the Copyright

• [ ] For each file, the copyright holders should be the people who have contributed meaningfully to this file (i.e. major contributors in AUTHORS.md). Look at the git history of each file to obtain a list.