Implement dynamic status response and data response

# Description

Currently, PyWPS uses static files for status response (e.g. http://localhost:5000/outputs/51eb13f2-7ea9-11e8-be8f-08002710f680.xml)

This works fine, as long there is no Exception stored in the file (`status=True`). If the client downloads the file, response code will always be 200. But we should implement correct status code returning.

Same applies for stored files: It should be implemented as service (e.g. https://apps.sentinel-hub.com/eo-browser/ uses WMS for data retrieving), since the data can no longer be only files, but e.g. stored in database.