Control which variable is written

Breaks default behaviour Updating project files may be necessary

• Only the variables listed in the prj file output section are outputted

  <output>
      ...
      <variables>
          <variable>X</variable>
          <variable>Z</variable>
      </variables>
  </output>

  A possibly existing property Y will not be written!
• In case of empty variables section all available variables are outputted

  <output>
      ...
      <variables>
      </variables>
  </output>

• OGS tools write all available properties to the result file

1. Feature description was added to the changelog
2. Tests covering your feature were added? - Updated existing many project files
3. Any new feature or behavior change was documented?

I'll clean up the git history after review.

ToDo:

• Tests with multiple output and different variable sections