update version numbers for v3.1 release

Merge pull request nest#425 from jougs/add_citations

Added citations for NESTML to README.md

Merge pull request nest#400 from DimitriPlotnikov/prepare_release

Prepare new release.

Merge pull request nest#371 from DimitriPlotnikov/master

Prepare release 2.1.0

Merge pull request nest#320 from DimitriPlotnikov/master

Print also spl warnings. Prepare release.

Merge pull request nest#319 from DimitriPlotnikov/master

Bugfix release: replace a class that is not available in openjdk

Merge pull request nest#318 from DimitriPlotnikov/master

Improve CLI output printing unit warnings.

Merge pull request nest#311 from DimitriPlotnikov/master

Bugfix: update CMake template to use the latest help generator

Merge pull request nest#310 from DimitriPlotnikov/master

Renamings of language constructs and predefined functions.

Merge pull request nest#298 from DimitriPlotnikov/master

Prepare bugfix release.