Code Overhaul

[davschneller]

We add two new features:

• The logger format was adapted to display the year and milliseconds, but not the day of the week anymore. It also optionally allows output from all ranks now—and it will print the rank where the info is from (possible overlap to fix logger to write for all ranks #5 ).
• The environment variables are now cached by default.

Furthermore, we format and tidy the code, and shorten the license headers to the necessary information [TODO: that'll need conformation if ok or not—all authors so far have been placed into the AUTHORS.txt file]. Also, we implement the REUSE specification for this project now.

The CI is also reactivated. Old functions (which are now in the standard) are deprecated.