Add DXF extension

[woutergd]

This PR adds the DXF output format extension to the WFS service. I already squashed the commits to a single commit. See also issue #531.

Known issues is that two tests are failing, but these were also failing for example on this PR: #541

[groldan]

Hi @woutergd, thanks for the contribution. Looks good.
For the sake of completeness, would you mind adding both gs-dxf-core and gs-dxf-wps to the wps-service? looks to be equally straightforward, just need to add the dependencies and the corresponding locations to the WpsApplicationConfiguration's @ImportResource annotation.

Also, have you, or can you, sign a GeoServer individual or corporate contribution license agreement?
Plase check the CONTRIBUTING.md file for instructions, as we hope and invite you to continue contributing to the project :)

[woutergd]

@groldan thanks for your feedback, I have updated the PR, and squashed already the changes. I haven't signed the contributor license yet, will make sure that that is submitted today.

[woutergd]

@groldan Just to let you know, I just submitted a signed contributor license agreement.

[groldan]

@groldan Just to let you know, I just submitted a signed contributor license agreement.

excellent, thanks a lot.