Releasing and tagging

We started using a tag-based approach to handle which commit is used in the stable KNIME versions that are currently supported. Usually KNIME offers support for the latest two minor versions (e.g. at the time of writing this is 3.1 and 3.2). Since GKN may become incompatible between two KNIME versions, we need a way to make sure that the correct GKN versions ship with the KNIME Community Contributions. That means if there is a new release here, test it with the current and last KNIME version and upgrade the tags to the last release that was working for that KNIME version. Use tags that correspond to the numbering on the KNIME Community Jenkins Server: knime-${KNIMEVERSION}

The develop branch should always be compatible with the latest KNIME version since its HEAD is used for the trunk builds of the Community Contributions.