While I am generally fine with contributing code under a permissive
license to other projects, I prefer strong copyleft free software
licenses for my own project.
By forking the `calendR` package I took over the maintenance burden for
this new project. This means I will invest more time into `calendRio`
than I would have in `calendR`. Thus, I prefer my work (and its users)
to be protected by a more restrictive license.

The licensing terms of the upstream `calendR` package explicitly grants
'the [right] to [...] sublicense [...] subject to the [...] conditions
[that] [t]he [...] copyright notice and [the licensing] notice shall be
included in all copies or substantial portions of the Software.'
These conditions are met by the header comments of all files retained
from the upstream `calendR` R package.
Additionally, this is clarified once more in the README Markdown file.
Furthermore, this licensing change was discussed and sanctioned by the
maintainer of the upstream package (see
https://github.com/R-CoderDotCom/calendR/issues/15

).

The `NAMESPACE` file (as well as the entire `man` directory) have been
deleted and re-generated via

```sh
Rscript -e 'roxygen2::roxygenise()'
```

Thus, they have been derived from source files (now) licensed under the
terms of the GNU Affero General Public License Version 3. This requires
these files to be distributed under the same licensing terms as well.

Since this leaves no single file left under the original MIT license
used by upstream, the REUSE specifications demand the removal of the
corresponding license text file from the `LICENSES` directory.
Note that the license test is still available via the README Markdown
file as well as the header comment of every file originally released
under those terms.

Unlike the (ambiguous) `MIT` license, the `AGPL (>= 3)` does not prompt
for a ` + LICENSE`-suffix in the `DESCRIPTION` file. Therefore,
`devtools::check()` expects not to have a `LICENSE` file present in the
top-level directory. While removing it would be in line with the REUSE
specifications as well, GitLab (and GitHub) uses this file to determine
the project's LICENSE. Thus, I decided to keep a symbolic link to the
GNU Affero General Public License Version 3 and add it to the
`.Rbuildignore` file to hide it from CRAN.

While some minor files that are part of `calendRio` are released under
the permissive terms of the GNU All-Permissive License, I decided to
mention the GNU Affero General Public License Version 3 in the
respective header comments as well. This is to stress the fact that
while the permissive terms stated in those files do apply to those file
specifically, the entire project is subject to more restrictive
licensing terms.
Note that this is redundant due to the REUSE conformity. However, REUSE
is necessarily well-known enough to avoid confusion without these
explicit human-readable notes.

Signed-off-by: Marcel Schilling <foss@mschilli.com>