Merge pull request #318 from beatrycze-volk/update-version

[MAINTENANCE] Update versions for update to 6.1.2

Merge pull request #311 from beatrycze-volk/update-composer

Update versions for release 6.1.1

Merge pull request #290 from beatrycze-volk/61

Prepare release 6.1

Merge pull request #269 from beatrycze-volk/release-5.4

Adjust versions for 6.0.0 release

Rise version number

Merge pull request #186 from slub/documentation

Enhance documentation

Merge pull request #159 from albig/fix-deprecations

Fix deprecations

Merge pull request #143 from albig/master

update documentation for release 5.0

Merge pull request #125 from albig/master

Update documentation and fine tuning.

Merge pull request #108 from albig/master

rise version number 4.0.1