[Improvement] Make indentation consistent across project files

[mvolikas]

What would you like to be improved?

In #1716 we noticed that some POM files were using tabs for indentation while others used spaces. The width was also not consistent.

How should we improve?

• Update the checkstyle configuration so that it checks the POM files. The default is to check only Java files:

    mvn help:describe -Dplugin=org.apache.maven.plugins:maven-checkstyle-plugin -Ddetail | grep -A5 "includes"
    includes (Default: **\/*.java)
      Required: true
      User property: checkstyle.includes
  

• Use EditorConfig and the available maven plugin to make sure we use the same width across files of the same type.
• Fix the currently inconsistent files.