Add license headers

[GreatEmerald]

GPL licenses suggest adding a license notice at the top of each source file, like this:

<one line to give the program's name and a brief idea of what it does.>
Copyright (C) <year>  <name of author>

This file is part of Foobar.

Foobar is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 2 of the License, or
(at your option) any later version.

Foobar is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with Foobar.  If not, see <http://www.gnu.org/licenses/>.

This is where it is explicitly allowed to use later versions of GPL (the main LICENSE text doesn't specify it). So it's a good idea to add these headers to each file to make sure it's really GPLv2+ and not GPLv2 only.

Relates to #4 as the header can be added along the Roxygen comments.

[u-ways]

I am going to do the R source files and create a PR for it.