Resolve "Add ewoks support"

Closes #26 (closed)

1. I converted the methods in darfix.core.process to ewoks tasks
2. Workflow files (*.ows) can be executed without GUI (i.e. using ewoks) from the command line:

   darfix -wf orangecontrib/darfix/tutorials/darfix_example2.ows -fd orangecontrib/darfix/tutorials/ -td /tmp/darfix

3. Added tests, documentation and notebooks

Darkfix does not make use yet of the widget base classes from ewoksorange. That's for another MR.

The ewoksorange widget classes do lots of boilerplate stuff like threading and using the ewoks task classes to execute the work (currently the task execution is still defined in two places: the widget and the ewoks task class). The ewoksorange widget classes also allow for easy testing, so there is no need for OrangeWorkflowTest anymore.