`mvn clean` should delete `node_modules` (1)

We should make sure that at least mvn clean gives a completely clean build environment. Therefore, we should use the maven-clean-plugin to delete the node_modules folder if it exist.

Update the pom.xml to delete the node_modules folder according to https://maven.apache.org/plugins/maven-clean-plugin/examples/delete_additional_files.html

Edited Jul 20, 2018 by Remco de Man

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